I got the first Skylanders day 1. I'll admit I had a lot of fun with it for a while. One thing I like more about Skylanders is how the characters level up and you can actually buy new abilities for them (an aspect DI is sorely missing).
When DI came out, my wife and I decided to quit Skylanders cold turkey. We even bought a bunch of the Giants figures, even though we didn't finish the first game. Like I said it was fun for a while but it definitely got repetitive fast, especially not being able to jump the world felt so flat.
The equivalent of "adventures" in DI was also kind of annoying in Skylanders. Each figure had their own adventure but you had to complete it with every single figure to boost their stats, which got extremely repetitive.
Lastly, Skylanders definitely feels a lot more kiddish. I know my wife and I are adults but we are having a lot of fun with DI and it doesn't feel like it is talking down to us as much as Skylanders did. Plus where Skylanders was a breeze, some of the challenges and adventures in DI are actually quite... challenging.
As for people saying the DI playsets are all the same, I don't think that at all. We beat MU and just started the first 10 min of Incredibles and they feel completely different. Skylanders levels were pretty but all felt the same and had the exact same gameplay.
So they are both fun games but we just enjoy DI quite a bit more. You have more iconic characters and worlds and collecting stuff is just a blast. I just hope they add more customization to each figure like Skylanders. Even if we did enjoy Skylanders a bit more (I know they're adding jump ability in the newest one), we're not made of money so it had to be one or the other we decided. And I think we made the right choice as far as which franchise has longer legs.