Milkbaby pretty much nailed it, but from my personal experience my Salsa Campion aluminum with a more aggressive geometry and a 39/53t x 11/26t gearing is a blast to ride and I view it as my "sports car", and I tend to ride it in a more sprited and aggresive manner whether I intend to or not. My Cannondale Synapse CF with a more relaxed geometry and a 34/50t x 11/28t gearing is more of a "sport tourer", and often 30-40 mile ride end up being 60-80 mile rides because I'm just having too good a time. If I had to choose one but I could keep my mountain bikes I would keep the Salsa because it makes me smile more and I love the speed, but if I could only keep one bike I'd keep the Cannondale because it is more versatile.