Dave, ur truck looks friggin awesome!
I'm also in between the 305/55 R20s and 285/55 R20s. I've been leaning towards the 285s for a couple reasons.
1) i heard the fuel milage will take a noticable hit with the wider tires, can you give any insight to this?
2) the required trimming would be less since the actuall sizes are 305=>33.19" and the 285=>32.28" for the Nitto TGs. BFG lists yours as 33.4"
3) I really didnt want the tires to stick out past the fenders. Wheels i'd get are +18 offset w/ 5.75 BS, i think stock GMs are around +35 which is maybe 5.6" BS (maybe someone knows for sure). how close are your tires to being flush with the fenders?
All in all considered, the 305s i think would look better, so if my thinking is wrong on those three points, i'd want to go with the 305s. Funny thing is, i think they're actually $20 cheaper/tire. Probably because of the load range rating difference.
Thanks