Vex seems nice, if you have the money (although a bit weak in some parts, considering the price), but you know that allready.
Their gears are nice, but plastic (barring the first gear on the motor axle). Their wheels seems good.
You could use just their wheels and gears and what you'd save that way can buy an awful lot of plywood, bolts etc. and it's not hard to make a simple platform for testing programming.
Yesterday I started such a simple platform to test a few things - A bit of 6mm plywood scrap, about 8" square (from some stuff I'm installing in my GFs kitchen) and 2 "furniture rollers" (don't know what they're called) in the front and I'm doing the remaining wheel as time allows (it's the one I want to test out).
Plywood is fast and easy to handle, cuts and drills easily and is fairly strong for the weight - almost as easy as assembling Vex or Mechano, but you are more in control over the final outcome.