The Berghia are cool, but it's not a good idea to get a couple to rid some pesky aiptasia. They'll not have anything to eat after the aiptasia is gone, and then you'll have a slowly starving nudibranch on your hands. Not to mention I think some nudies can crash a tank when they die, I'm not sure if it's the Berghia, but I've heard it before.
A peppermint shrimp is IMHO the best solution to the problem. They prefer the smaller ones, so they don't let them get big enough to release plague proportions of their planulae. To get rid of the others just mix up some decently thick Kalk and spread it over the oral disc out to the tentacles.