If I were to give a quick and dirty comparison of 3d World vs the Galaxies it would probably be something like, the Galaxies have the greater highs but 3d World is much more consistent.
Which shouldn't be too much of a surprise. Galaxies Achille's heel (if it has one) is how much and often the games moves through various gameplay types. Its to be expected that not all of these different gameplay styles would be equal in quality. For me, I thought what the games achieved with gravity and perspective easily surpassed any levels in which they weren't prominently used. So most any of the Mario64-like stages (Throwback Galaxy, Beach Bowl Galaxy, Honeyhive Galaxy, ect.), 2d sidescolling sections, or even levels like Bowser Jr's Airship Armada and Toy Time Galaxy would usually take me away from the number of thing I loved doing in the game. Not only that but moving across so many gameplay conventions, you could sometimes be stuck doing an action or playing a scenario that didn't promote platforming or ,even worse, platforming being replaced with an action that wasn't very dynamic or fun to use (Bee and Boo Mario say yo).
For 3d World I feel that "2d Mario feel in 3D!" is the core of the play experience (my favorite 3d Mario feel). Its a philosophy that extents to movement, pacing, power-ups, level design, you name it. And its a type of play that is carried much more strongly across most(all?) of 3d World's varied (but perhaps not as varied as Galaxy) scenarios.