Both are meant to firm up the body and decrease body flex. That is keep the car stiffer and therefore handle better and take turns harder and faster.
The front bar doesn't really affect the body flex up front because, heck there is a big engine there and alot of weight for one bar to stiffen.
The rear does seem to make a difference because its the only thing back there to stiffen it up and since there is not much weight back there, there is a noticeable difference.
Both are meant to do the exact same jobs, only in two different places. One seems to make more of a difference than the other.