Neither will *need* spacers assuming a normal tire size (sub 34") but may benefit from spacers depending on what tire size you choose. But that's an odd selection for sure. Basically the most off-road option and the most road-only option out there. Opposite ends of the spectrum. What are your goals for the truck? What kind of driving do you do?
IMO, the 21's are for pavement use only and not the best size for ride quality or tire choices. If you like the "big wheel" look, I think the 20's that came with your truck (or some other 20" wheel) would be a better choice. Many go with the variety of OEM 18" options for a balance of off-road capability, on-road performance and ride quality. The 17" Rock warriors are a great wheel too, but getting harder to find in good shape and kind of expensive. 17"s will have a less direct on-road handling feel due to the taller sidewalls, but it's all about driver preference. I love my 17's.