Of the two you've linked I'd have to say the Cannondale, biased? yes I am, but the CAAD frame was the best alu frame you could buy in it's day, and is only now surpassed by later CAAD versions.
Yes you could get carbon for your budget, but FWIW I would personally never buy a second hand carbon framed bike, (the seller isn't going to admit to crash damage are they?), and secondly I wouldn't buy a new one either with your budget. Rule of thumb - get the very best frame your budget will allow, and with all due respect to your budget, that won't be carbon fibre.
The most important factor is fit, so get around to some shops and try as many different bikes as you can for size, that way you can be assured you're getting the right size as well if you do buy second hand or elsewhere online.*
*don't pi$$ off your LBS's by tyre kicking and wasting their time though, (unless you genuinely intend to buy from them of course), but I personally would have no issues in doing that in one of the big chain stores as they don't care about you as a customer anyway, whereas there's a good chance your LBS will.
