I would choose MLB 08: The Show over MLB 2K8, just for the simple fact that the Sony studio put so much effort into this year's MLB game.
Here is what IGN said as the closing comments:
Last year, PS3 owners were let down by strange AI problems, weak visuals and game features that just didn't deliver what they promised. That's not the case this time – MLB '08 delivers on every gameplay front. The on-field play is excellently done, the stat tracking is immense, and Road to the Show is one of the best career modes around. Visually striking, this is the baseball game that baseball fans have been waiting for on the PS3.
9.0 Presentation
MLB milestones, radio calls, deep stat tracking, replay vault and revamped visuals across the board makes this one of the strongest showings of The Show yet.
8.5 Graphics
Loads of new animations, cutscenes, redesigned character models and parks shows what an improvement this is over last year's game.
8.5 Sound
The addition of your own songs to the jukebox is a nice feature – too bad they're only restricted to the menus instead of the park speakers.
8.5 Gameplay
Many of the AI issues from last year have been addressed. Pitching and Batting analysis tools are a great addition, although there are still some fielding problems.
9.0 Lasting Appeal
Road to the Show is easily the best career mode out there, and should keep you extremely busy when you're not playing online.
8.7
Great OVERALL
(out of 10 / not an average)
Source: http://ps3.ign.com/articles/856/856121p3.html