You could use either one, but the 7.3 would be easier on pavement. How tall are you? Is a 17" frame your size?
I don't think I'd be wanting to spend quite that much on a used bike, though, unless it was in really good shape.
I thibk the fx is better deal, as long as the frame size is right. Get to 1 or 2 trek dealers and see whst size they would fit you.
If the size is not right you should keep looking for the right size or just buy a new bike. There are some good ones at around 400. Usually when you buy new you get some benefits like feee adjustments for a while