The Galaxy duo edge out the 3D pair for me, I very much enjoy 3D World for what it is, in fact I just finished another run through like 30 minutes ago and while in actually tackles my biggest flaw with the rest of the 3D series (that being a chunk of powerstars devoted to minigame like tasks that can fluctuate heavily in quality) it doesn't quit hit the highest marks due to its hybrid style of 2D and 3D Mario.
Control is interesting in that it can go both ways, the swifter and snappier feel of 3D World is something I much prefer, Mario can feel a touch sluggish when starting his movements in the Galaxy games by comparison. Galaxy does however have more fun acrobatics to play around with as the base Mario, you can get more immediately creative with the likes of triple jumps and air spins, 3D World tends to require suits (most specifically the catsuit) to reach this same level of innate playfulness, while there's a number of various jump types available in 3D World as well it seems like a number of them are kind of redundant hitting similar heights or distances to more easily used moves, I feel in this respect Galaxy's various acrobatic moves have more use as a whole.
3D World gets some stronger replay variety courtesy of differing characters and the various suits available while Galaxy tends to use its suits/power ups more strongly regarding level design since they're directly built into the progression. Bosses are more varied and interesting in Galaxy as well even if they still remain pretty easy throughout.
Varied level design is a strength of Galaxy 2 and 3D World in particular, both of these games tend to have many ideas that are one and done, each stage offering something new or an inspired spin on something introduced earlier in the game, Galaxy and of course 3D Land with its revamped post game falter a bit by comparison here.
Music is an easy nod for the Galaxy pair, the big band style of 3D World has its moments but I don't find the soundtrack as consistent nor hitting the highest marks that both Galaxy games reach.
When it comes down to it Galaxy's main strength to me is that its more suited to being a 3D game, bar a few staright up 2D sections in both games it really continues to take advantage of platforming in a 3D space, the 3D series will often have sections that while 3D feel like they'd work just fine in 2D.
On top of this the multiplayer focus hinders some level design, underwater stages are basically 2D with a bit more leg room not even able to tackle to wider expanse 3D Land covered, things have to be reigned in more to accomodate multiple players, that being said the mp is great fun aside from the odd choice of player pickup button.
While I'm at it there's my more personal little bugbear that while 3D World is visually polished an bursting with colour the environments themselves come across like vaguely themed props to the platforming within, It's all well and good showing me Bowser's neon skyscarper land but that's really more of a background visual dressing while I jump around geometric brickwork in the sky, Then there's that opening stage of 3D World's World 5 which is an open beach stage that's basically a big rectangular block of sand and shallow water, it's like a double whammy of bland visual design and multiplayer constricting the level design.
The Galaxy games kind of get away with the similar issue because the setting of space and planetoid platforms give it a greater sense of place where the open void of clouds and space just fit a lot better as well as offering its own unique points due to the gravity element, with 3D World/Land we're basically back to the floating Diorama worlds that SM64 often stepped into, at least the platforming can take advantage of this freedom.
Short version is basically the Galaxy duo have a grander audiovisuals, take greater advantage of powerups to strengthen level design and are more well adapted to a 3D space. 3D World still has boatloads of creativity and fun, its just that the hybrid 2D-3D style leaves it falling short of those extra strengths a game committed to either angle can attain. The Galaxy games managed to hit a sweet spot between the wider exploration and openess that 3D affords while still fitting in obstacle course pieces, 3D World does a fine job as a more direct set of obstacle courses itself but I just think that NSMBU had that covered well enough already.
...sudden realisation that I'm all but arguing the difference between 10/10 and a dreaded 9.5.
