the ps2 has alot of good games at the moment that any age can appreciate, the wii, has alot of "shovelware" or, rubbish that people make for it in a month because its selling well. I would keep the ps2 for now as there are so many good games. The wii still has very good games but, sticking with nintendo games only will mean you wont get rubbish (most of the time). There are ALOT of bad wii games so don't let your kids pick anything up if it looks ok and make you foot the bill. i would suggest using
www.metacritic.com, search the game, it will give the average score of all the proffesional reviews on that game, so if something is a 70 etc you will know it will be ok.
So, in conclusion, don't be hasty to sell the ps2, there is alot of good stuff on there and the wii seems like it has only a few games that are really worth the money each year. When your son doesn't play on it for 6 months, then sell it