I say 12v. What are the "real world" advantages to going with 24v?
Faster line speed....is this really an issue?
1/2 of the amp draw...is this really a problem? The cables aren't THAT big.
Physical space (smaller motor)....are you that tight on space?
Less weight (smaller motor)....are you really that close to your GVWR?
Needless complexity.....do you really want it? Every moving part is one more possible failure in the field when you least expect it, and can least afford it (like in the middle of the desert, solo, and your rig finds a soft patch of sand and proceeds to sink).
IMO, there is no question. If I were building a competition rock buggy....Yea, I'd look at 24v because time really is money, but in your (or my) situation, there is no way I'd seriously consider 24v (unless you answered "yes" to the above questions...in which case I *might* consider it).