I've tried both BeerSmith and iBrewmaster on Mac OS and I ended up staying with Beer Alchemy (which is still quite a nice app despite lacking features).
iBrewmaster is hopelessly buggy at them moment. If it gets fixed in the future, then maybe OK but as of 1.0.8, its not worth the money.
BeerSmith is very comprehensive but I just couldn't live with the interface. As a Mac user, you get used to that look and feel. BeerSmith has none of it. Yes, it works on Mac OS X but it's like trying to do a full boil on an electric stove top...
Mind you, Beer Alchemy is like that at times
I've been using Beer Alchemy so that's probably the main reason for staying with it. If I were a new user, maybe go BeerSmith as it's far more comprehensive at the moment. The next release of Beery Alchemy looks very promising though.
Like Mike said though, download the trials.