They are both good bikes, very well built and reliable for every day use. The 8 is more relaxed geometry than the CAAD 10, slightly more compliant in its tube profiles. If you can get a 2013 CAAD 10 5, the 105 components for the mechs and brakes for that price, new I'd say stretch the budget to get it. The CAAD 8 with Sora is a good entry level bike but the 105 components, on the CAAD 10 5, are very good, light and well engineered mechanicals for the price. There is a CAAD 8 with Tiagra components, as mentioned above, those are a level below the 105 in Shimano's line up and are good components, slightly heavier and are basically the older 105 design. That is the CAAD 8 to look for, in my opinion, too. The wheels on the CAAD 10 are lighter, higher level wheels, but are lower spoke count than the CAAD 8 wheel sets. The CAAD 8 wheels are good commuter and entry level items.

I am biased towards the CAAD 10 as I own a 2012 10 4 (Rival group) it has been a great bike all round, ridden every day without any mechanical glitches what so ever in the 2 years I have had it (4000+ miles last year.) My only changes to the 10 were to go to Continental Gatorskin tires, 700X25 size when the stock Schwalbe rear got a stone cut that ruined it, too big a cut to boot or try and repair, and add a K-Edge chain catcher as a precaution. It is a light weight, slightly stiffer and aggressive frame and wheels that is able to be upgraded as you progress in your skills and fitness, easily, as high a level as you want to spend for.

Best of luck in your decision, either one won't disappoint you at all. Do some reading in the Cult of CAAD thread in this forum for some ideas on how they are both thought of here.

Bill