BeerSmith is the only software I have actually tried. I downloaded the demo and loved it. I did take a look briefly at ProMash, but it looked to me like that program is old and it didn't seem to have been updated in quite a while based on what I could find. I just double checked and it hasn't been updated since March of 2003 unless the website just hasn't been updated to reflect that. That tells me that the program is rather dead, even though it may work fine. It's just not being updated and enhanced as time goes by.
My recommendation would simply be to download the free trials of both and see which one you like best based on your own preferences
I'm a Mac user, so BeerSmith is a no go for me. I downloaded the demo of Beer Alchemy and have been pleased with it. I'll probably end up buying the full version.
In my opinion, this question is a bit like asking which is better- glass or plastic as fermentation vessels. It really depends on you as a brewer and what you want to get out of the program.
I use a Mac so BeerSmith was out for me; that left me with BTP and Beer Alchemy. I started with Beer Alchemy and found the ingredient database and limited flexibility in constructing mash schedules a bit disappointing. BTP has a somewhat steep learning curve and has an obvious bias towards all-grain brewing where BeerSmith is more accommodating for extract + steeping grain and PM recipes as well.
I use beersmith. I thought about trying promash for fun as well. From what I have heard though, some of Promash is out of date and has not been updated in quite some time.
I used Promash for many years. There is another thread about this, but I went to Beersmith like many others because it is more recent and is being updated more regularly. In addition, it handles batch sparging better (more simply). It seems to me to have more information regarding efficiency rates and I think it is easier for someone that is new to understand the great brew checklist that is prints for you.
I'm a two-year BTP user that only briefly tried beersmith on my first two all grain batches. I really have been meaning to try smith again now that I know what I know. The thing is, any software change is hard even if the newer one is better.