I own both, even helped in the development of the last ProMash version on the testing team a few years ago. They are both good, useful programs.
The ProMash interface seems to be a little overwhelming at first, but when switching to Beersmith I experienced the same feeling. I feel that Beersmith takes less time to get used to, though.
Beersmith has two advantages over ProMash.
First, it is in active development and support by its creator(s). The creator of ProMash seems to have disappeared from public contact. So if you lose your registration information, or need some other assistance, you may not get any help for ProMash. You will for Beersmith.
Second, they have useful forums to get help from other people as well as the Beersmith people, who often jump in and help there.
Some things seem to be easier to do in ProMash, some in Beersmith, but they both have so many features you would be served well by either program. Both are available as tril download versions, so try them both. At least ProMash, after the trial period is over with, will retain some useful utilities, so you may want to keep the trial version even after it expires if yo go with Beersmith.