St Sheldon endorses 3/32" because it's lighter and (he claims) strength is not really an issue at the kind of gear ratios used by road and track fixies---a granny gear on a mountain bike puts many times the stress on a chain that a fixie rider could muster up. I believe him, but my drivetrain odyssey started with a 1/8" Surly cog so, you know, things kind of went from there.