It's really going to be six of one / half-dozen of the other. However, if it were me, I'd go with the Blast over the 4500. Since you're a West Coast rider, I think you'll find a west coast bike more suited to you. On the whole, Treks are set up for East Coast, tight woods, pick-your-way-through riding and the frame geometry shows in a steep 70.5° head tube angle. The Blast has a more relaxed 68.5° HTA that is going to be more adept and confidence-inspiring (IMO) for the terrain we have out here.