I am a devoted user of both iBrewmaster and BrewPal.
I think BrewPal is infinitely more adjustable and customizable. Many aspects are far more user friendly. Finding certain items to play around with, however, can be frustrating as there are almost "hidden" menus. Plus, as a brew day app and recipe creator, it doesn't have a stand alone equal. However, it IS buggy.
As a long term recipe database and for a much less cluttered interface, iBrewmaster is more superior. I think BrewPal is more useful on brewday, but if I'm using an established recipe, I almost exclusively use iBrewmaster. It's less capable, but less of a headache.