I currently use both BeerAlchemy 2 and iBrewMaster. Here's my opinion, for what it's worth, BeerAlchemy would be my perfect iPad brewing app if it wasn't for one exclusion, Timers/Alarms for mash and boil/hop additions. This exclusion is the only reason I currently use iBrewMaster as my main brewing app. However, I mirror everything I do in iBrewMaster to BeerAlchemy in hopes that someday soon it will include the timer/alarm function. iBrewMaster is very hard to master and the UI is fragmented and not well thought out in my opinion. You'll definitely need to spend a few days with the app and also refer to the manual many times. Looks like a hard-core windows or linux user designed this app. On the other hand, BeerAlchemy is very much Mac like, the workflow is easy to understand and use and not fragmented at all and the layout logical. There's a free trial Mac version. To me, a very useful thing with BeerAlchemy is I find it easier to design recipes from my iMac and keyboard. I also find it easier to enter batch notes, etc from my iMac. Once I enter them from the Mac version of BeerAlchemy, I just sync with my iPad and everything, including brewing notes, inventory additions/subtractions, new batches/recipes are now on my iPad. Lots of nice calculators as well.
Both will do the job and both are great programs. I'm a mac user and just find iBrewMaster very hard to navigate and the workflow is very cumbersome and fragmented. I've heard a major rewrite of the software UI is in the works and at that time I might change my mind, but for now, I highly recommend BeerAlchemy.