It's not quite like either of them, actually.
The physics are based on the DS game which is itself a heavily modified version of the SMB1 physics but with jump heights more closely resembling SMW.
If you played DS you'll be familiar with the extra moves; wall jump, wall slide, & ground pound.
One of the really cool parts is the new abilities they gave Yoshi. He's more or less like the SMW version, but with the addition of his YI flutter jump, the ability to grab hammers and spit them back at enemies, & he instead of getting powers from colored shells he gets them from the enemies he grabs. For example, I grabbed a venus firetrap and when I spit it back out Yoshi instead spat one of those big fireballs from "Mario Bros.". I don't know if he can get any other powers from enemies, but my point is that there is a crap-load of cool stuff in this game.