I'm sticking with V-tech ever since my kid was born
Right now he has the Mobigo which is also a good "learning toy" even though it's more of a Console than a Pad (even if it has touch-screen display)
Vtech mostly sticks to a full game software principle. Those apps are sold for roughly £15 to £20 each.
you can connect them to your computer and have a parental panel that shows you the learning curve and the achievments.
you can also download smaller apps for roughly £1 or £2, at least we can for the mobigo, I don't see why you couldn't do that with the pad (better double check though)
Leapfrog works more like your regular pad, and seem to have more apps, but leapfrog never impressed me enough to buy one of their products.
I've tried them in store and was underwhelmed.
Some other DISers might have an opposite opinion and experience