ALL-TERRAIN BIKES: If sitting upright is important for your comfort, consider this category. All-terrain bikes, also known as mountain bikes or city bikes, are the fastest-growing type and currently claim 35% of the market, largely because they are versatile, durable and fun to ride. They can be raced in off-road competitions, loaded with bags for long tours or used simply to get around town. Their extra-wide knobby tires and higher bottom bracket -- the juncture of tubes at the pedals -- make for easy riding over rough territory, giving extra protection against bumpy trails as well as potholed streets. The upright position, plus brakes and gear shifters on the traditionally shaped handlebars, are comforting reminders of childhood bicycling for many purchasers. ''It's the yuppie bike,'' says Edward Schwinn, president of Schwinn bicycles. Happily, as ATBs have turned into hit sellers in recent years, attracting more makers, prices have plunged. You can buy roughly the same mid- range quality ATB for $300 now that would have cost $700 only four years ago. For the most part the more reasonably priced choices, such as the Specialized Streetstomper ($325) or the Univega Rover Sport ($330), are general-purpose ATBs meant to be ridden casually on the road as much as off.