I'm a newbie to aero type wheel sets and I just bought a used wheel set advertised as a 2006 Zipp Clydesdale 404 Wheel set. The price was good but I just that realized the front wheel rim says 404 and the rear says 909 - they look the same with regard to rim depth so I can't figure out the difference? Is this a good combination? or is it better to have two 404s???