More seriously, they're pretty wildly different. Art style is nothing alike, for one; SMRPG goes for an isometric pre-rendered look, which is pretty nifty, while Mario & Luigi goes for a more traditional pixel art approach with a cartoony style, which I'd say is the more effective of the two. SMRPG lets you have three party members out of five total (Mario, Bowser, Peach, Geno and Mallow... although I don't think you can swap out Mario), while Mario & Luigi pretty much only has, well, Mario and Luigi, and places a lot of emphasis on what the two brothers can do as a pair, with lots of team-up attacks based on jumping, hammering and moves based on their different "elements" (with Mario focusing on fire and Luigi electricity, IIRC). Both have a lot of focus on platforming outside of the battle system, with the ability to skip normal fights by avoiding enemies entirely, and feature the whole "Timed Hits" thing in-battle (hell, SMRPG had the feature from the get-go, and I wouldn't be surprised if it introduced the concept outright). The plots share that balance of humor and seriousness that all Mario RPGs tend to have. I think SMRPG takes itself a bit more seriously, but only just; it's kinda hard to assert that a game with a wedding cake for a boss is "serious", y'know? Aurally, they're both composed by Yoko Shimomura, so no matter which game you choose, you annoy Dark Schala get about the same great level of composition quality.
So... yeah, looking back on it, they really are pretty similar. M&L has a lot more surrounding the whole gimmick of having Mario and Luigi together at almost all times, while SMRPG's gimmick was being an RPG starring Mario (unheard of at the time!), and as such plays the concept a lot more straight. Both are worth a look; it just boils down to what you're in the mood for at a given moment.
(Note that I've only played Superstar Saga, so what I've said about M&L is generally limited to that title.)
