Heh, ah, to be more accurate - the five types are, from slowest to fastest:
1) Treads. These things will carry the earth but you will move as slowly as someone carrying it, too. They have little/no upkeep and only use energy/heat when you move, so that's a plus.
2) Legs. Support a decent amount. I actually haven't taken a close look at their stats - I forgot what makes them special. They don't move too quickly though, it's usually (100)+.
3) Wheels. The only ground-based prop mod that can get you under (100).
4) Hover. Unlike the ground-based systems, more hover mods = more better. These are starting to have a noticeable active upkeep AND move costs for both energy use and heat generation. They support a fair amount, surprisingly.
5) Flight. God tier, but I'm biased because I always run light. These things barely support themselves and consume exorbitant amounts of energy and heat, but move incredibly fast. Three flight mods and you are moving a disgusting (20), but not for long because energy/heat problems. Also, you can jump over units! This is totally abusable - if three happen to be in a row, you'll move over ALL THREE. Amazing. If you go overweight though, you're basically better off crawling.