The n54's were in the 07-10.5 years and the n55's were in the second half of 10-current. They are both pretty potent, however the n54's have been known to be more tuner friendly and more along the lines of a SD tune where as the n55's are MAF(IIRC). Also the n55's came with a single twin scroll turbo and the n54's are true twin turbo. The easiest way to figure out which 2010 car you have is to look at the engine bay. If it says on the cover "twin power turbo", then it's an n55. I'd lean toward the n54's myself as it's been rumored that 2010 and forward didn't come forged from factory like the previous years, but that hasn't been completely confirmed. Another consideration is the 335is and the 1M coupe have n54's and not the newer n55's so what does that tell you? Also try to find one that hasn't been tuned. It's hard to know for sure, but if you read the instructions on how to install a jb4 or proceed you will see the parts that come off. Look at the condition the tabs and screw are that need to be removed to install. They will be more worn than a regular car.
EDIT
Not sure if you are interested in the sedan, but here is some more info. The LCA(facelifted) sedans came out in 2009 so if you want the nicer looking sedan and the n54 you will need to look for years 09-10.5. Not sure if the coupes were LCA, but the sedans make a big enough differene to make me lean toward one. I have an 09. I also have the AWD which is known to be hell from a dig, but trap a few MPH lower than a comparably modded rwd varient. You can also get the AWD in stick, but they are a bit more hard to come by. If I could do it again I would get a stick AWD 09, but the steptronic is all good.