As of this writing, there are only two baseball games available for the PSP. This is the one you should not get. Although I have never played MLB (the other game), I can tell you right now that is the baseball game you should get if you are debating over the two.
Let's start off with the good. Of course, the graphics are amazing. The players actually look like themselves. The gameplay is also decently smooth, with easy-to-understand controls. Instead of pressing X to throw and then the direction pad to a base, each button throws to a different base.
The game doesn't have dynasty mode, but it does have season mode, as well as quick play. Dynasty mode is when you can control your team for more than a season and you can control the team's finances. Season mode is the full hundred-and-something games, with the ability to trade, change your batting order/pitching rotation, and basically everything else except anything to do with money. Quick Play is basically like exhibition mode.