I'll 2nd what Roundslide said, it's all a matter of personal taste and riding style. I run a 17/41 now and it's just about perfect for me, and even 2 up with my wife. I tried an 18 and I didn't like it. Lost too much acceleration off the line.
It's cheap and easy to try out front sprockets. They're only $20 each plus shipping from Brent.
