All of the above. Mario and Luigi would probably beat Paper Mario by himself in a swimming competition though.
I've always preferred the Mario and Luigi games for several reasons. Luigi gets to co-star with Mario probably for the first time since the original Mario Bros. Practically every other game, Luigi is absent or you use him instead of Mario. This is a good setup that allows more extensive character interactions with each other, and yeah, the game does deliver on it and appears to be even more on that dynamic in Dream Team (though I feel it's pretty significant in Partners in Time too). Paper Mario falls short here. There's just so much more you can do with Mario and Luigi compared to Mario and an underbaked partner that eventually gets replaced by other partners or loses most of its character once the chapter the partner is relevant in ends. I also feel Mario in Mario & Luigi is more involved in what's going on there rather than just staring at what others are doing, but Mario & Luigi isn't immune to it either, because of those accompanying annoying characters like Starlow.
I also prefer Mario & Luigi's art style too. I have a preference for more on-model depictions of Mario and friends. I do like a lot how Paper Mario is depicted (more than years ago but I think this has to do with how I used a Paper Mario over Olimar mod as well as using Paper Mario character in MUGEN), don't get me wrong. I just like the formed legs, the eyebrows, the jet-black pupils, and the expressive faces Mario makes. Bowser looks less puffy as he does in Paper Mario, Peach has prettier eyes, Dry Bones, Boo, and Hammer Bro. look less strange by being closer model to the contemporary designs) (not this weird ass Dry Bones human skull from Super Mario World, not Boo's weird upper lip from the Yoshi's Island games, not these weird "Koopa wearing gear" of Hammer. Bro.). Some people do like the diversion from the contemporaries because they add a degree of weirdness and helps make a game appear more unique, and that's fine, though! I just prefer more subtle arrangements like the black irises or the slightly lankier proportions of Mario in Mario & Luigi Dream Team and beyond.
I do like that Mario & Luigi is built upon having existing characters getting new roles. The first game, as I said, is about Luigi actually simultaneously being with Mario and being able to do cartoon mishaps to each other to advance to new locations and beat enemies. But the second game gives us a rare starring role to Baby Mario and Baby Luigi. The third game also gives us yet a new co-starring role with Bowser, which is the closest we got to a Bowser game in the entire franchise. The fourth game didn't really do anything like the first three, and it was disappointing because it seemed to be a good time to give roles to Waluigi and Wario, especially given with that they did to Baby Mario, Baby Luigi. But in the fifth game, we got the Mario & Luigi world and the Paper world clashing, and it led to ridiculous and entertaining scenarios. Maybe it fell short in giving us both elements from those series, but it could've went wrong very easily and I still think with what we got was super interesting and pulled off pretty well. Paper Mario is capable of pulling off neat story lines, but they seem to do best when they're classic and straightforward, and I wasn't really pulled into any Paper Mario's story set-up as I am to Mario & Luigi. Mario & Luigi just seems to be better at using existing characters than Paper Mario, as Paper Mario marginalized Bowser well after the first game (to be fair, Mario & Luigi Superstar Saga didn't treat Bowser all that well), Paper Luigi just seems to be "there" and has a weird personality clash with the rest of the series, and there's not much other existing characters like Toadsworth, Nabbit, E. Gadd that Paper Mario uses that Mario & Luigi doesn't, though I have to give credit, they did use Piantas. Peach might be the only character that's better than the Mario & Luigi counterpart
The one thing that Mario & Luigi suffers from is that they have an ally that is, well, insufferable: Starlow. I don't know what the developers see in her, but I don't like her and she's wholly unnecessary, as we've seen from how people played the Superstar Saga remake and remarked how "refreshing" it is to feel that Mario and Luigi are really the ones in charge. Mario & Luigi also seems to rely on gimmicks a little too much, which does negatively affect the experience; Bowser's Inside Story has egregious touch screen minigames, Dream Team has the atrocious broken gyro controls that are *almost* as bad as Quick Time Events). Mario & Luigi also never took the opportunity to develop Firebrand and Thunderhand, which is very disappointing because electric and fire elements certainly add more to the mix of dealing with enemies.
I didn't really go into the gameplay sort of things because, to my embarrassment, I haven't played either games extensively though I did watch my sister and brother play those games, but I don't have much of a preference. I like Paper Mario's elegance in how damage is dealt and given. Mario & Luigi has a more traditional system of SP that lets me pull off flashy moves, but I do like their method of relying on cues to try avoiding damage.