3.21 and 3.92 are the rear gear options with the 8spd. 3.55 is the option for the 6spd which is still found in some classics.
Personally having owned the 8spd and the 6spd, I'd say get the 8spd all day. Smooth, great highway mileage, and reliable.
Also my 5.7 Sport has 3.92s, shes faster off the line than anything that size should be. Also need 3.92 for max tow, so if you plan on a trailer above 7900lbs you need that.