Leapster!
Our family has a v-smile, v-smile pocket, 2 leapsters, 3 Nintendo DS (I'm a pack rat

)
Anyways - I found the v-smile was great with my kids were younger. The v-smile pocket a little harder to hold and use however. I keep it around in the playroom for daycare kids, and sometimes my 6 year old will still play with it.
The leapster keeps kids attention much better! The games are more interactive - they can trace/draw letters, color and more. AND I found the older kids (ages 7-10) will still sometimes play with the games as the game cartridges are designed better for older kids. A lot more games in the library for the leapster as well!
By the way, at 6 and 4 they WILL like and use the leapster games! But be warned... if you get your 6 year old a Nintendo DS - that's what the 4 year old will want. Using the leapster (a particular game cartridge) taught my son the alphabet and how to write his letters before he entered JK.
My 11 year old hardly ever touches the leapster now since she was given a Nintendo DS. My 6 year, unfortunately, was given a Nintendo DS as well... big mistake... he would have kept playing with the leapster a lot longer. In fact, sometimes I "lose" his NDS so he'll pick up the leapster again. Educational, but still fun is the leapster.
Dani