6.8
18" barrel only because it makes it a little easier to get the end of the barrel out of the hunting blind without having to stretch out as far. Otherwise 16" is plenty, the extra fps you'll get from anything longer won't make a difference till you get out beyond 350 yards. The same goes for the 6.5 Grendal, it only surpasses the 6.8 in the 400 yd range and beyond.
My advise is to do some internet searches, there's some good charts comparing the 6.5g vs the 6.8. Hint - the 6.8 actually does better within 300 yds. Also, if your building the upper check out
http://ar15performance.com/home for your barrel. You'll see them routinely mentioned as ARP on various ar forum sites as some of the best barrels for the money out there.
Also, if you don't reload go to your local shops and see which caliber has the most ammo readily available. Unless you'll be shooting past 400 yards the deer won't care which caliber you chose.