20" PROS:
- Lighter (for the same wheel in 21")
- More selection in wheel styles
- More selection in sizes (widths and offsets)
- More selection in tires
- Rides more cush/comfy
21" PROS:
- Fills up the wheel wells nicely! (Looks better)
- Can have a step-lip without making the wheel look small
Basically, 20" CONS = opposite of 21" PROS, and 21" CONS = opposite of 20" PROS.
What really made me decide on 20" was the tire sizes. I think the sidewalls for the 21" rims are too small and will bend the wheel. Ride will be more harsh also. Also the 305/25/21 MPSS are discontinued and the 295/25/21 are back-ordered.