Something else to consider it whether or not you'll need to get on top of the layout, e.g. if it's against a wall and you need to reach in corners. My old tables were built with 2x4s (mainly because I had them available from a walls I took down in the basement) and I could certainly get on top of it. I don't think I would climb on if it was supported with 2x2 legs. My next layout I dont' plan on having to get on top so I'll use lighter construction.
Don