My daughters have the leapad and I've seen the innotab, the leapad is of a much higher build quality. It can take a good beating and both my 2 and 4 yr old play with it a ton. My 2 year old loves the drawing program that comes with it. The software is pretty disappointing IMO for the leapad. It uses the same cartridges as the normal leapster explorer, which means the game are not optimized for the system, at least as far as I know. They are also pretty expensive, although there are frequent deals for them.
Honestly, I think you are far better off spending a little more and getting an 8 gb iPod touch. Sure, it costs more up front, but the software is way cheaper, usually at a buck, cannot be lost(unlike a cartridge), and there are way more games. My kids like their leapads but absolutely LOVE the iPod touch.