Guys, just a point to think about, in Israel we don't have "locol Radio Shacks and Home Depots".
In my opinion vex is the best system from the above list, although to really build a cool robot you need to buy a lot of expansion packs and it can get fairly expensive. I was lucky and was able to bring a lot of vex stuff to Israel, so Leav if you have a way to do so then get vex.