The Leappad has a larger screen. It also has a motion sensor in it so you can tilt or turn it to play certain games. There are games that only work on the Leappad and can't be played on the Explorer, but all Explorer games work with Leappad. The Leappad also has a microphone that the Explorer doesn't have.
The Explorer has an available charging base, the Leappad doesn't (but you can use regular AA rechargeable batteries in either device).
The Explorer battery life is longer than the Leappad (smaller screen requires less energy). You may be able to find a gently used Explorer, but are unlikely to find a used Leappad.
I would lean towards the Leappad for the larger screen and camera. There is a camera available for the Explorer, but it is a $25 addition and by the time you have paid that much you might as well have the Leappad where everything is built in.
My DD4 has an Explorer she got for Christmas last year. She is asking for (and getting) a Leappad this year. If you can find it, Toys R Us has a bundle for $99 that includes a game and app card for the Leappad. It is only available in green and is very hard to find (they sell out almost immediately).
