-between those 2, I'd go with the 2011 if it's within budget. The 2011 with I5 has a quad-core processor (vs dual I3), a faster video processor probably more VRAM that's GDDR5 vs GDDR3, and also offers 1 Thunderbolt port (vs just a mini-display port on the 2010).
-Thunderbolt could allow you to buy a Thunderbolt dock for some extra expansion if you really wanted to (like adding USB 3.0 ports), though even a bit more money maybe you could get a 2012 model which offers built in USB 3 ports
-so long as you get a 2012 model with at least 8 gigs of RAM (the 2011s were the last model with (reasonable) user replaceable RAM) The 2012 loses the Firewire port though, so you'd need a dock/adapter to connect any Firewire peripherals
-I recently bought used, and for me it was the toss-up between a 2011 or 2012 - re: RAM, whether I would miss the Superdrive, and USB 3.0 - I went with the 2012 for the USB 3 and being 1 more model year better (and the guy was throwing in the Apple external Superdrive
)
everyman comparison page between 2010 and 2011 model years:
http://www.everymac.com/systems/app...n-imac-aluminum-mid-2011-mid-2010-models.html