[Peck]

first: I (like others who have posted before) would recommend at least a decent amount of experience on your team before dealing with more then 2 axis (x linear and z rotational [or equivalents]) in your drive train. At the very least, make sure someone who has done it before is helping you as it can get very confusing very quickly if u don't.

on topic: what is better depends on the intended use

mechanum is poor at climbing due to the rollers so it has to be significantly taller then whatever u intend to climb. It also stinks at countering gravity. general robustness is well below average for the weight involved. The only real advantage of the system is that it is highly mobile (capable of motion in x linear, y linear, and z rotational [or equivalents] all at once or independent).

finally: if u use it at a comp, bring at least one full set of spares since it takes time to repair and time isn't usually on your side (Murphy's Law is ever so true).