I've played more of Kingdom Rush on my iPad than any other Tower Defense game I've ever played - It's absolutely fantastic.
The one problem with the iOS version was that (as is sadly standard for the platform) it had microtransactions - there was a store that let you buy consumables (for gold you earned in-game or purchased) to use in levels, as well as one-time purchases to unlock Hero characters. The Steam version fixes this, and has no DLC or microtransactions at all - all the heroes are included in the game, and it doesn't even have the consumables store anymore.
The Steam version also includes a bunch of the bonus maps/mini-campaigns added in updates to the iOS version after launch, with the promise that the rest are coming soon in a free post-launch update that will also add a new bonus campaign exclusive to the steam version.
My only small disappointment with the steam version so far is that there's no way (that I can tell) to add a keybind to call the next wave early. There does seem to be some sort of nonfunctional keybind-manager in the game launcher, so hopefully that's something they'll add soon. (UPDATE: This is no longer true - there's now a keybind for this!)
Kingdom Rush is, in my opinion, the best TD game out there, and this Steam release is the definitive version. At $7.50 it's a steal (I can't tell you how many hours I've sunk into this game), and I strongly recommend it to anyone who even remotely enjoys TD games.