with the right gearing you should be fine with a ss. if you're a runner you've probably got the leg strentgh you need.
gears are great too though. you'll definitely go faster with them, provided you know how to use them.
oh yeah, i was the dead weight on a state champion cross country team in high school, so i have a little familiarity with your situation. i'd say go with whichever you think will be the most fun to ride.