I own both Beersmith and ProMash. But I only use Beersmith now. The top three packages all have most of the same basic features (e.g., recipe calculation, brew day scheduling, inventory management, etc.), but there are a few things about Beersmith that make it stand out for me. First, it is also the only one that seems to be under ongoing development (ProMash hasn't been updated in years, and BeerTools Pro still isn't 'finished' yet by its developers). For example, it is the best software out there for calculating batch sparge infusion schedules, it does a great job of recipe calculation for late-extract additions, it can handle bitterness calculations for first wort hopping, etc. Furthermore, the developer of Beersmith, Brad Smith runs a great support forum and he is ALWAYS on that thing answering questions, taking advice for future features in the software, etc. Finally, I personally think that Beersmith has an intuitive and very user-friendly interface (unlike ProMash -- but I believe BeerTools Pro is good in this respect too).
Far and away, buying Beersmith was the best $20 I spent for my home brewery. Very highly recommended.