I am trying to decide which bike to purchase and am choosing between the Gary Fisher Piranha or Trek 4900. I've looked at the specs for both of them, and it appears that the components on the 4900 are a small step up from the Piranha. I want something that can ride around town and can also take me on some intermediate mtn. biking trails. I don't really care about the grip of the tires on concrete, but any other aspects that you feel important would be great. Also, how do the two different forks differ? The Piranha has a Manitou Axel Elite w/TPC Lock-Out and 100mm travel, while the 4900 has RockShox Judy J1 w/preload and 80mm of travel. What is the difference between 80 and 100mm travel? How does the geometry compare or differ on each bike? Please give any of your opinions on either of these bikes, whether you own one or have test-ridden one. Please give your opinions.

Thank You

One last question. If I need a 17.5" frame and I can get a 18", should i get the 17.5" or the bike with the 18"?

I'm 5'7" and 135