if the trek 4500 fits you the best go with it. its a decent bike and fit is the most important consideration for selecting a bike. if the Piranha fits you, that is a great choice, and in my opinion worth the extra money.

have you considered Kona, 2006 Cinder Cone retails for $699 (you may get some discount from the local shop). Also, the Blast is $599. I know they are down-speced from 2005, but I find Kona frames absolutely amazing. Kona frame construction, handling and durability is top class. give it a try if you can and want to.

Avalanche and trek 4900 is also pretty good. I really liked Diamondback Response Sport 2006. its around $400.

sorry for adding more confusion. best of luck.