It also depends on the amount and style of riding. Aluminum frames, while heavier, tend to have a little more flex than carbon. So; if your riding long distances at a moderate pace, aluminum might be more comfortable. If you want to ride against yourself for time or just speed in general, carbon might be the way to go.

Remember, components make up a lot of the bikes performance- I know that's a little like Master of the Obvious, but a great frame with a substandard gear set/derailleur is no bargain.

TREK makes a halfway decent bike for $699, Litespeed has some good products- although not cheap.

Local shops CAN be good, but REI seems to have the most knowledgable and consistent advice.