I'm getting back into biking after a long break and I'd like some advice. I'm primarily going to be using my bike for commuting to work (15mi each way combination of rural highways and city) and casual road rides, so i was planning on getting either a road bike or a hybrid because I keep reading about how they're faster than a mountain bike. For a commute like mine, is it really going to cut down on my time that much? I'm trying to stay under $500 for the bike because I don't have much money to begin with, so that limits what I'm able to buy, but I'm wondering if it would be worth getting a decent mountain bike rather than a bottom-rung road bike or hybrid for the same price. Is it primarily the weight, the tires, or something else that tends to make road bikes and hybrids faster in the first place?