I'm in the market for a bike and had narrowed it down between a mountain bike and comfort bike (leaning more towards the comfort bike). Then I came across a couple of really nice looking 21 Speed Schwinn Hybrid bikes. One of the Hybrids has fenders (good for all weather trips) and a rear bike rack. What are some of the "main" advantages/disadvantages of a Hybrid bike?

Bike will be used for excersie and running errands around town. I weigh approximately 230lbs (but shrinking) .