Usagi and Minako are not the same

So there's this narrative that goes around the SM Fandom that goes something like:

"Naoko Takeuchi originally did not plan to make a team based MG series and started with the solo Codename: Sailor V before Minako was added in after she was asked to make it a team series instead. She based the main characters of both series, Minako and Usagi, after herself which is why Usagi and Minako are almost exactly alike"

There's a major problem with this theory however; mangawise Usagi and Minako are not actually that similar.

On the surface it seems like they are similar characters. Usagi and Minako both look alike and both have magical talking cats telling them what to do (the reason this is the two of them is for in-universe reasons). Both are lazy (and do poorly in school), eat a lot, play video games as a hobby, and are considered "unladylike". But such is a very shallow reading of their characters.

Codename Sailor V, Chapter 1

Bishoujo Senshi Sailor Moon, Act 1

When we first meet Minako she's doing her gymanstics

when Artemis accidentally gets in the way causing her to rage and say she'll "kick your *ss" to the person who put a cat there

before Minako goes right back to being cheerful and energetic giving a peppy introduction of herself to the reader

Hikaru mentions that Minako is a show-off and we also learn quickly that Minako is confident in love, enough to give love letters to the objects of her affection. 

Artemis who has been watching comments she has excellent athleticism and an appetite to match. We also see her sleeping in class and getting into fights with the teacher

 

this leads into the page shown at the beginning of the contrast, where Minako says that waiting around for love isn't her style and she's gonna go and tell him forth right. This in turn causes some of the boys in class to be afraid of her and how she doesn't act like a girl and Minako sticks out her tongue and them and says she never cries (crying being an obvious symbol of a girl's emotionality and her sentimentality)

In contrast the first image we see of Usagi is of her panicking and running off late to school

Humorously enough Ikuko, Usagi's mom, mentions in passing that Usagi isn't a thing like Sailor V (who is actually Minako)

This leads to page at the beginning where Usagi introduces herself and we see quickly that she is a self-described crybaby, that she likes to sleep and she tripped over a cat. Her reaction to tripping over a cat is very different the Minako's and is notably sentimental

As Usagi gets in trouble we are introduced to Usagi's immature habits that get her in trouble at school like showing up at late (from oversleeping), eating when she isn't supposed too and not studying

Interestingly Usagi comments that Naru is the "perfect lady" being really smart and really pretty, as well as very classy indicating her embarrassment at her own lack of development, supposed by how she is never shown to be actively arguing of these people about their claims (though she does mentally complain how unfair it is that Mrs. Haruna makes a sweet girl like her stand in the Hall) .

Usagi and Minako circa chapter 1 are not only not really alike, they are at times diametrically opposed.

Minako is an energetic peppy girl who has a lot of self confidence and no problem arguing and fighting those she doesn't like. She is identified as being unladylike primarily due to her lack of emotional sentimentality or romanticism.

Conversely Usagi is presented as a somewhat withdrawn girl who avoids confrontations, is a lazy crybaby led by her emotions. She is identified as being unladylike primarily due to her immaturity and lack of composure. 

Minako and Usagi both initially resist becoming soldiers of justice after their first fight but for entirely different reasons. Minako claims the outfit is "breazy" (thought she learns to like it later) and that she wants to get paid for her work

Usagi on the other hand didn't want to because it scared her

Minako never expresses fear at fighting, her reaction to being attacked by a horde of basically zombies (included being grabbed by one from behind) is annoyance

and in chapter 4 when Petite Pandora starts stealing all hot guys she transforms into one using her magic to lure Petite Pandora into kidnapping her so she can surprise her.

Minako is so unafraid of fighting that one time she literally just forgets to charge her weapon 

Needless to say, Minako is not presented as being cowardly like Usagi is. 

Another point people like to bring up about Usagi and Minako is that both play video games as a hobby. However Minako loves video games, and even before getting trained by the Sailor V, was apparently good enough to get the second highest score on a popular game.

Artemis takes the opportunity to reinforce again that Minako acts like a guy.

Conversely Usagi went to the local arcade mostly to see Motoki who was her crush at the time and is implied to be pretty bad at games, losing to a "wimp" boss.

Granted, Minako did have more practice on that particular game then Usagi did but in general Minako actually seems to like games, bringing them up a bunch while Usagi only occasionally mentions it in passing.

And while both do poorly in school, it's implied that Minako is actually smart but is just lazy and doesn't do the work like she's supposed too while Usagi does poorly at school just because she's honestly below her school's average intelligence, though this is a high-bar considering her school is implied to be pretty hard.

In general, Minako is rather tomboyish, confident, peppy and confrontational, while Usagi is a very immature feminine, not confident, lethargic, and avoids confrontation. It's not like their character arcs any closer. Minako's character arc in Sailor V is the dawning realization that to do what she has to do, to protect people means giving up her actual desires and her want to be fun. As the Goddess of Love, her ability to actually find love for herself must be sacrificed in the name of protecting love for others. Upon her full awakening as Sailor Venus, bringing her back all her memories including the tragic destruction of the Moon Kingdom and Death of Princess Serenity, Venus finally cries

That she has to sacrifice her desires and her chance at love to do what she has to is why at the end of the series to stop the Dark Agency and protect the World she has to kill her own true love, who prophecies that Minako will have to continue to fight on forever

and why she says her form as Sailor V is her true form, and that her life as Minako Aino was merely a disguise until she was awoken again

Usagi's arc on the other hand is quite differently.

Usagi spends the first arc of Sailor Moon having identity troubles. At one point she tries just telling her dad that she's Sailor Moon but he doesn't believe her and she wonders which is the "real her" between Usagi and Sailor Moon.

And after she awakens as Princess Serenity, Usagi comments that it feels like her identity as Princess Serenity is dissolving her identity as Usagi. 

Usagi is struck trying to resolve her triple identity as Princess Serenity (the Past Her who suffered tragedy), Usagi (the Present Her who is unsure of herself), and Sailor Moon (The Ideal Future Her that can be a hero to everyone), culminating in the battle against Queen Metaria where Tuxedo Kamen tells her that he will always be her at her side

Seeing this man she has known as Prince Endymion, Mamoru Chiba, and Tuxedo Kamen pledge his support for her through time, the man she loves across identities allows Usagi to finally unite herself, unified by her defining characteristic, her love.

Usagi then announces herself as Sailor Moon and Princess Serenity, having unified her identities through love

Usagi and Minako don't think the same or act the same and their character arcs don't start or end at the same place. 

So why do people think they are similar. I think it's a combination of their superficial similarities and the changes made by the 90s anime. The 90s anime made Usagi more bubbly, peppy and snappy while it made Minako calmer and more passive and classically "feminine". As a result the two ended up seeming very similar. In the manga however it's very clear cut and writing in their voices shouldn't sound the same at all.