The biggest downside was SMG's easiness. But even with some harder levels I just wasn't having much fun because there was no reward except getting a star to levels.
SPM is RPG/Platformer so its more fun to me. It brings back the nostalgic feeling of side scrolling mario game. But sadly its also extremely easy (so far). The level designs are as easy as they come. Also alot of things can be passed extremely easy with flipping to 3d mode and just walking by.
For example the fire things that used to be in Bowsers castles:
Alot of the levels, you can put 3d mode and just walk past those EASY. So there are no real obstacles. The hardest parts are kind of a puzzle when you have to figure out how to do something. Though those are usually easy to.
So since both games are really easy, it comes down to the Nostalgic feeling I get from side scrolling mario and the fact that it has a RPG aspect of leveling up, items, abilities, etc.