I think you would struggle to find a brand new Tiagra groupset bike with carbon for that price.
Maybe, but I was always told to buy the very best frame your budget will allow, everything else is easy to upgrade as you go, (if you want to of course).
So with that in mind I would never buy a carbon frame with the OP's budget. I'm not saying a carbon bike at this price point is junk, or it will fail, or anything else for that matter, just that a carbon frame in this price range will be the lowest quality carbon construction available, whereas an alu frame won't be, certainly in the case of the CAAD8 anyway, that was, and arguably still is one of the best alu frames money can buy.
Yup I'm biased to CAAD's I admit, but not without good reason.