I think 20s are too big IMHO, 18-17 is more my preference. I've read several complaints of a stiffer ride with spacer kits, and have felt the difference in my brothers truck. Granted he has the regular oem Chevy shocks, after his 2" spacer his truck road much worse. I read and read and read, and decided i would either do shocks or just go all out and do coilovers. I went with Bilsteins and granted it cost me 700$ (shocks + install) vs 350-400$ (RC 2.5 + install) i'm very satisfied. Had i had the money i would've grabbed some dirt logic coilovers, for a 2" lift.
Though i think some of depends on the shock/spacer combo. I don't find that my cousin's 09 sierra with 295/70/17 Toyo MT and a 2.5" RC level rides that rough. My brothers however does.