I applied the equations in
this whitepaper to JVN's Calculator (with some other mods as well) last night and came up with some interesting calculations. Over lunch today I further tested things, and have reached some conclusions:
A 5-lb minibot can reach the top in under 6 seconds
A 10-lb minibot will struggle to reach the top under 10 seconds
A 15-lb minibot probably won't make it to the top under 20 seconds ("probably" because there's a 10% fudge factor here)
There are only 5 gearing/wheel combinations that will even lift a 10-lb robot to the top in under 15 seconds, and all of those are well behind any of the 5-lb robot combinations.
Thus, the lighter the better.