What are you going to do with it?....plink steel and target shoot, hunt 2 or 4 legged creatures, etc.
How far do you plan on shooting it?
Are you a reloader, or do you buy commercially avail. ammo?
In a military world, a 5.56 LP(light precision) rifle like the MK 12s allow you to carry more ammo/mags and maintain commonality with the M4 platform in case you need to borrow additional ammo, mags, spare parts like a bolt, etc. Generally speaking the 5.56 rifles have a much better track record for reliability in the field as well but the MWS and EMC are certainly changing that as we speak.
A 7.62 is a lot harder to "drive" than a 5.56 in a precision capacity.
You need to use a narrower brush my friend