I have played every single Mario & Luigi game except for the BIS remake, while I only played SPM for the Paper Mario series. So, you can see why I would want to prefer M&L over PM. M&L has good gameplay no matter the game. And the only exception is found exclusively in the remakes: the Bowser's Minions and Bowser Jr. modes. It was a cool idea for Superstar Saga due to how little Bowser's Minions appear through the game despite appearing at the beginning. But I feel it feel flat. SPM has gameplay that is only found in that game. It was a platformer RPG. From what I see from videos of the first two games, I see good gameplay in those. Sticker Star and Color Splash, on the other hand, had different gameplay which is a little faulty. I mean, you need to use something just to jump or hammer. Next: story. The first three PM games have good story with the third being a little controversial. The latter two had little story, but then again, Color Splash did try to add more characteristic to plain characters. The M&L series stories are great in Superstar Saga and BIS, while DT and PiT are good as well as PJ, but it was Sticker Star characters meet their 3D counterparts for the most part. More on PJ later. I feel that M&L does have better gameplay and better story.
In terms of which should have the next game, both seem like good candidates for to do so, but PM is more likely. First, there is the fact that it hasn't gotten a game in a while and second, there is the fact that AlphaDream went bankrupt. But we won't know which of the two will get the next RPG until it comes.
Now onto Paper Jam. It was a crossover of characters from Sticker Star with the Mario & Luigi universe. So, the gameplay is the similar to the other Mario & Luigi games. I don't really consider it being a Paper Mario game, but that doesn't mean it shouldn't be excluded from the Paper Mario (series) page on the Mario Wiki due to the elements in the game. But it is a Mario & Luigi game nevertheless. It is a good game, but I wouldn't say it is better than Superstar Saga and BIS. I say that despite story being somewhat worst than DT and lacking a separate gameplay (inside Bowser in BIS and inside dreams in DT, exclusive to those games), it is still better than DT with its better basic gameplay and shorter distance from the final boss to the place where you can access anywhere and some better improvements. As for PiT, I say that it is definitely better in all areas except maybe story. Overall, I think it was a good game despite what people say able it. But yeah. It isn't as good as Superstar Saga or BIS.
...hmmmmmmm