Hi, I guess you could easily call up a dealership that has both items in stock, talk them nicely and have someone pull them from the shelves and do a direct comparison for you.

However, I have seen many of the Fender Extension Arch Flares listed as for 20" and 21" wheels. I guess one of the key differences for the wheels besides the obvious diameter could be the Offset or Stickout which you can increase with Spacers.

http://www.ebay.com/itm/131076146925...%3AMEBIDX%3AIT

http://www.ebay.com/itm/Genuine-BMW-...820991&vxp=mtr

http://www.ebay.com/itm/BMW-X5-E70-0...11df16&vxp=mtr