CAAD8 5 105 or CAAD8 6 Tiagra?

Not much difference. The CAAD8 5 upgrades you to 105 shifters and derailuers. Since both are 10 speed set ups ether can be upgraded as parts wear out. Ride both and see if the shifters feel any different. Since the 105 system has been upgraded to 11 speed (105-5800 series) you should be able to get a really good deal on the CAAD8 5

Thank you, Which one would you buy? Which one is more prone to resisting rocks and other things I can find on the road? Which one is easier to upgrade/fix?

^There's absolutely no difference between the two in any of those areas.

I'd agree you'll be hard pressed to tell much of a difference, especially if this is your first road bike. As Mr645 mentioned, the new 105 groupset is out so you should be able to get a great deal on the 5 with old 105 group. For the small price difference (which will be even smaller if you find/work a deal), I'd go for the 105.

With that said, if this is a first bike, what kind of riding do you plan on doing? Although the CAAD8 has a slightly taller head tube for slightly more upright and comfortable and seating position, but it otherwise has the same geometry as Cannondale's race bikes, which might mean handling quicker than you'd like and less stability than you'd want for a first road bike.

We're not going to be riding this bike, you are! That said, the most important thing you can do is to go ride each one and see how they feel. While the differences are negligible, the most important factor is how they feel under you.

Would you buy a car simply on stats, knowing that you'll be spending many hours behind the wheel? I hope not.

Same is true of a bicycle.

Good luck!
Brian in VA

Other than the shifters/derailleurs, the bikes are spec'd virtually identically. They both have [censored] wheels. My two cents, get the Tiagra model, and put the couple hundred you'll save towards wheels down the road. Ride the Maddux ones until you start snapping spokes, then invest in better ones.

You really won't notice much of a difference in shifting performance between Tiagra and 105.