I wouldn't do the leapster 2... and haven't really seen the didj to know enough about it.
Last year when my daughters were outgrowing their pixters (they were 5 and 7 at that time), was looking at the leapster 2's, and seemed too "young" for them.
If I remember right, the age range for those is up to 8 yrs old.
Ended up actually getting them the Nintendo DS's for Christmas... along with 3 educational games each.. and they love them. Got them the brain age and brain quest games, and some math and spelling ones too... They've also gotten a few "game" games in the meantime which they love playing each other using the built-in wi-fi...