Try not to get too cheap a tyre ... you don't want a flat every 50 miles. Get one with some form of protection, like kevlar belting, etc. My slicks suck (Performance City Slicks), coz all the flats I get are on the slicks. My ATB tyres have never gotten flats yet (knock, knock). Also, be sure to look at the weight of the tyre. You want to conserve as much energy as possible so you can go as far as possible in one ride. Obviously the width of the tyre is directly proportional with its weight.