Here's a fun thought:
Disney Infinity: Take away the toy box, and what do you have? A rather boring platformer with a truckload of characters to play with, that doesn't really do anything different from each other. Violet turns invisible, Mr Incredible punches really hard and Mrs Incredible stretches her limbs, but other than that, they are all the same. All charactes level up to 15, but what's the point? They don't really get new powers or abilities, and don't get much stronger.
Skylanders: No toy box, but a much better platformer. Each character are very different from each other, and they all not only get a lot of skills and abilities, but their skill tree branches out in two directions, forcing you to pick just one direction to build your character. Not only that, but they originally only leveled up to vl 10, yet they still have a lot more skills and abilities than anyone in Infinity. They added five levels in Giants, but that's just extra stat points.
So all in all, Infinity would have been a terrible game if it wasn't for the toy box. Now imagine Skylanders *with* a toy box... But either way, it's all very simple:
Do you want the toy box? Go with Infinity.
Do you want a great platformer? Go with Skylanders.
Do you want both? Awesome, so do I.
In the end, think of it like Grand Theft Auto vs Saint's Row. The first Saint's Row was a copy of GTA: San Andreas in most ways. But after that, GTA has gone down the more serious route, while Saint's Row... well, in Saint's Row 4, so far I've become the president of the US, beat up some aliens, got stuck in a Matrix-world, blasted aliens with my dubstep-gun and have superpowers. Hehe. Disney Infinity and Skylanders might look a lot the same on paper, but they are two entirely different games.