I started riding a few months ago at about 255 lbs. and bought a Gary Fisher Nirvana. It's a hybrid and it has very sturdy 700x25 wheels/tires. I've had no flats and no weight related problems. I prefer the hybrid because the knobbly tires of the mountain bike are not very efficient on pavement which is my main riding surface. I occasionally ride on grass or dirt trails but I'm not jumping logs or roots, etc. The fact that the hybrid can go off road on reasonable trails makes the bike versatile enough to give me confidence to handle any situation that I'll need to deal with. Good luck. Riding has helped my weight and health immeasurably.