The bikes I see ridden by messengers in downtown Toronto are all over the place. It seems to come down to a few critical points: 1) what can you afford? b) what can you afford to lose? c) is it comfortable? There are lots and lots of guys that make it around on old hybrids and rigid mountain bikes. Almost none ride high end bikes, and that makes sense. Why would you wear on a nice bike for a job that can't really pay for the resulting maintenance?