Mew. It's far cuter. Such a simple design, but I would go as far to say it's still one of the cutest Pokemon, maybe even in my top 5.
It also felt like the first truly legendary Pokemon back in the day and I remember how hype and amazing it seemed since you needed to go to special events only in select places to get one (unlike now where there's so many of them and they're so trivial to obtain). It's definitely nostalgia as well as a prime example of Nintendo's manufactured rarity at its best but hey, it worked, that helped make it seem way more desirable, even now.
Mewtwo? Eh...
It's original Gen 1 sprite was just pure ugly (though most gen 1 sprites were). I hated it. It looked like he was straining to force out a poo.
I also remember only really finding out it existed after the elite four (that's how I remember anyway, I may be wrong and its mentioned during the game) so there was no build up to it, it was just kinda there, in a cave, like "hey, kid, there's another Pokemon, go get it", then I just throw a master ball at it and its done, caught, to no fanfare. It was just kind of the opposite of Mew hype to me, especially considering it's in the game, you can catch it on every playthrough, very obtainable, not really hyped up at all and chances are good you're probably just gonna throw a master ball at it.
Normal post-gen 1 Mewtwo is still """ugly""", but in a cool way. I wouldn't want to pet it but I would like to make it fight things.
Then there's just the general gen 1 oversaturation of which Mewtwo is one of the more popular Pokemon to be used as nostalgia bait. Regularly focusing specifically on Mewtwo in newer games (though the same can be said for gen 1 in general), he was unsurprisingly a big focus of Detective Pikachu, Pokken used him and his 'dark Mewtwo' thing as basically half their advertising strategy, Pokemon Go spammed Mewtwo everywhere in its advertising, Smash has Mewtwo, I don't recall as much fuss for any animated Pokemon movie as the ones where Mewtwo was back (in fact I was/am literally unaware to the release of any new Pokemon movies...Except those)...Then there's the overuse of that "what you do with your life" quote recently from that awful first Pokemon movie that I'm just sick of hearing/seeing...They're remaking the first movie too, of all movies they could have chose from, or just done a new one...Or just not. That's it, "just not".
I think generally Mewtwo is in the same tier as Pikachu and Charizard where I more hate the Pokemon because I'm just sick of seeing/hearing about it than actually having anything against the Pokemon itself.