I have owned 2 Mauls and a rapier. They are both good. However, for pking, the maul seems to get me killed a lot. People veng right before you hit and if you happen to hit high your basically dead. The other thing with the maul is it is really bad for combos. Think of trying to do some kind of switch with a godsword as your primary switching to dds or something similar. Rapier on the other hand is good for everything. It hits better than whip on high defense, and hits much harder. The only bad thing about it is if you pk with defenders, you will lose them, where as the maul you dont need one. Anyway I have done rapier to korasi versus other berserkers and it just owns. Throwing in a dds spec after you veng or hit high with rapier is really easy. Switching is a piece of cake because of the speed. So personally, I like the rapier more. Now if you had lower combat and turmoil it might be a different story. Also, rapier is godly for PvM. I used to never to slayer/PvM but when I got rapier I tried it once and it was so easy I do it every now and then.
Go for the rapier, its a whip on steroids.