We have both an explorer and a leap pad and can say they both have pros and cons - the explorer can have a rechargeable base station so just plug it in over night and it's good for the week. The leap pad can use rechargeable batteries, but they need to be removed to be charged and you have to get those on your own- otherwise it's an endless supply of dead batteries.
The explorer is smaller, but the leap pad has the camera built in. The explorer has a camera addition sold separately. It seems as though the leap pad will phase out the explorer - if that's the case the cost of the explorer may come down, but will become outdated quickly. As for the old leapster2 - it's harder to find games and is not being updated anymore. I'd go with the leappad
The games are great - the girls love them both (ages 2 - now 3, and 3 now 4.5) and play them both equally as much there are some apps that are only for one or the other (typically anything for the explorer works on the leap pad).
The games are challenging, but doable and grow with the kids. We have the Disney princess game, the princess and the frog, x-men, doodle burg, and have tangled waiting for a special occasion. The explorer also has the addition of the online world to explore - there are other games and help with mouse work nd keyboard use.
As for QWERTY, our girls use the computers often enough we are more interested in the other features.
bottomline: get the one you like the looks of better. They're both close enough with pros and cons for each. Neither does everything. If the leap pad had a rechargeable base, I'd say hands down get that - but the rechargeable base is a huge selling feature for the explorer IMO.