These are both out of date but if they are the only options, I'd go with 2011... the current server is server12r2 and that comes in standard or data (i.e single or multi licence).
I find server12r2 much better overall than any of the older systems and (for a server) it's muppet safe but the big question is what will this server do? Windows server essentials (aka sbs) was a pre-set package for people that wanted remote access on/ to a small (25 users max) system... remote access is very much an out dated product because the proxy server system has been replaced by remote management and most people just don't use it anymore so upgrades to a new remote access server will leave you locked into a system which will need more and more tlc just to stay a-float.
This server is being used by a medical institution so getting up-to-date is not that easy for them as their medical applications do not support the new Operating Systems yet. That's why we would need to stick with SBS 2008 or migrate to 2011 for now. So may we know which will be easier, migrating from SBS 2008 to SBS 2008 or migration from 2008 to SBS 2011?
Given those details; it shouldn’t make any difference as far as the actual transfer of files/ rolls.
As I already said, 2011 is slightly more up to date so I’d be inclined to go with that… the fact is I wouldn’t take on the job because the client is unwilling to upgrade their system correctly but that’s not really the point.