Cant comment on any of the others but my son has a Wii and the best things about it are
1) Promotes family style play rather than losing your children to the console - Lots of shared playing games and fun
2) Most games are pretty active - rather than simply sitting with a controler in your hand - you are jumping, throwing, punching and twisting about with the controller - Keeps the kids a little more active than most the consoles and again makes it a fun experience for all the family.
After buying my son the PS2 when it came out - I regretted it after a few days - as I lost him to the system. In the end, had to ban him from it - Just think they are not very sociable things....The Wii on the other hand is completely different.
Worth finding if anyone has one who you know and give it a try.....same with all the other consoles.