I've got the PSN download of MLB The Show '09. I played a friend's UMD version for a while before I purchased the digital download. The biggest difference I noticed was in the load times. It is noticeably better on the downloaded version. I would assume the same would be true for this year.
I believe you can have the digital version on up to five PSPs associated with your PSN account and I think you can download it on any of those as many times as you want. Plus I think you can back it up to your PC using Sony's Media Go app (at least the saved game data, not sure about the actual game).
And not to completely hijack the thread

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Since I haven't played '10 or '11 maybe one of you can answer this for me- does the new version allow you to play any minor league games in Franchise mode (or at all)? I'm struggling with the decision to purchase the new version, and having the minor league teams in would sway me.
I think MLB2K11 on PSP has them now, I thought maybe The Show would do the same. If they are, does the roster download include any minor league updates?