[MrForbes]

I would guess that using 4130 would allow us to use a thinner wall, for a lighter assembly with the same strenght. The downsides are that it's kind of difficult to MIG weld structural parts with less than 1/16" wall thickness, and that it's not available in town...which is a concern for us, especially during the prototyping phase.

I agree that it's wise to pick the best material for each part of the robot. There are a lot of things to consider....weight, strength, ease of fabrication/machining, ease of attaching other parts, cost, availability, repairability, etc.