I have both and the graphics are better for 2011 but I might actually think the overall game play might be better for 2010 but I have not totally decided yet. Keep in mind I have only owned both for about 2 weeks so I am going off a short sample. Also I believe you can play a lot more seasons on 2011 but I don't care about that since I only play one season at a time anyways and then change up my teams.
I have noticed a lot more homeruns on 2011 which I don't like. 2010 has an awesome option in my opinion where you can use a "heavy ball" which makes it harder to hit homeruns. I really love that option but some probably don't care. I will be curious what some of the PYS veterans on this site have to say about 2011 vs 2010. I am definitely not a vet, just started playing the series lately.