It all depends on the car's weight distribution and spring rates. I would think the bigger of the two is best if it is a solid bar, but it's just a guess. The bars are not the only thing to consider either, as chassis tuning requires a systems approach. Remember that if it understeers, you need a bigger rear bar. Oversteer you need a bigger front. That is unless you choose to change spring rates instead as roll stiffness is what determines the car's tendency to do either. Then again, weight distribution also plays a critical role as well, but that is getting into things like polar moments, etc. which is complex and hard to quantify here. Plus, I haven't got that far in my math and physics yet and can't really explain it well.