Diamond Mind Baseball is a very good simulation. It has a bit more depth and control than PB(maybe too much, but that is another discussion), is a little cheaper, and allows you to replay seasons on your own, as well as combine seasons, and play old seasons.

Diamond Mind is meant for season replays. You can be in leagues if you don't mind delays in standings, stats and updates, and don't mind keeping track of your teams at bats and innings.
Each league has its own commish, # of teams(10 up to 24) and set of rules(maintained manually). Realtime is also out.

PB is a better game for leagues, stats updates, the format, injuries, realtime, trading, and all things are done online, under one set of rules which are monitored by the computer.

If your into leagues, I couldn't imagine switching from PB to DMB. Buy DMB to replay last year or other years, you can play it along side PB, but don't dump PB for DMB if you enjoy team ownership in a league environment.