What is the difference between all the Leapfrog devices?

Kidsandliz thought this was worth mentioning said
5

My mom said she’d buy a Leapfrog for the 2.75 and 3.75 year old (boys) except she knows nothing about them and neither do I. I was surprised that there were a bunch of different model choices. Can anyone tell me what the current best choice is out there for pre-schoolers, and which educational games, etc. are best for these two kids? Thanks!

PS if anyone has one their kids have outgrown that isn’t too old, doesn’t need the battery replaced, etc. and games/ed stuff for it we’d be willing to buy it used from you. I have yet to see any on our local craigslist.