The CAAD 10 is a completely different frame than the 8. Not much weight difference and the ride is probably negligible for most people, but an entirely different frame with a full carbon fork (be that good or bad).

If that's a 2013 CAAD 10 it's a nice price and you probably don't have to worry about availability waiting (though they do sell quickly in some areas.)

Do you need the 10? Probably not but resale will be better down the road.

Tiagra vs 105 - Lighter weight with the 105. Looks different and I personally think the 105 shifts better. If you've never had it you'll never be disappointed. I friend of my has Tiagra on his cross bike (several years old) and it works just fine. He's had no problems and again, it's an older version that has been improved over the years.

Get the one you like as long as it doesn't put you in a financial bind. I just bought a CAAD 10. Wanted an EVO but couldn't justify the expense. I could have gotten a SuperSix, but prefer the CAAD under my weight. One day I may justify an EVO, who knows:-)