Chain vs. Belts?!?

[asid61]

Originally Posted by Knufire

The pulleys that VEX and AM sell are 5mm pitch HTD, so that's what most FRC teams use. Usually 9mm or 15mm wide. You can also run GT2 tooth profile belts on HTD pulleys.

If you use a GT2 profile with HTD pulleys, it's not supposed to add any benefit, so generally speaking it's better to just stick with HTD for availability purposes.
The lower limit (and it's a sketchy one) for 5mm pitch 9mm wide HTD belt pulleys is 24t apparently for a typical year (read: not this year). I believe that 36t is safer than using 24t, especially if it's your first time. If you use 15mm wide belts I'm not sure, but 24t is supposed to be safe with those.
You would be better served asking "Chris is me" because I'm just parroting what he says at this point. A quick search on google should turn up some relevant threads about belt sizes.