Go for the one that fits, with the right geometry for you - the one that feels right. However I don't think I would buy a secondhand carbon bike. Most manufacturer warranties apply only to the original owner, so to assure yourself the carbon one hasn't been abused I'd be looking for scuffs etc that might hint at a bingle. A new bike will at least give you some free servicing (usually) and a warranty.

I did buy a used alloy bike on ebay years ago because I couldn't find one my size to try; even had my bike shop check it over as fine. But discovered later it had a split head tube and could only salvage parts.

Good luck!