first things first, how much are you trying to spend and do you have an idea of what year you want? and i guess secondly are you even looking at a diesel? i just jumped to conclusions and figured u were
personally, if i were you i would buy a 2006 or 2007 classic chevy duramax paired with the allison...i remember looking at this exact truck when i was purchasing in 2010 and simply couldn't find any that hadn't been sold yet...i liked the chevy better because it looked better than the GMC, i thought the GMC was ugly although there isn't much difference other than that...since you like chevy already this is what i would do, however i do like the looks of the 2007.5 and newer better but i would only go that route if you bought a gas engine...
for what it's worth, i ended up with a 2008 super duty power stroke...absolutely no regrets, now that i have all the emissions deleted