The problem with 21" wheels is you are extremely limited to tire brand. Dunlop SportMax RFT are the only choice in recommended size. Going to 22" there is a better choice of rubber and really you won't feel any difference in ride between the two sizes.
2013 X6M Sapphire Black-current
2013 Audi S4-current
2007 BMW X5 4.8i (E70)-sold