I"d say thinner, because that will allow more reps, and you can always make it thicker. You can put PVC pipe ofer it, you can hang from two rolling thunders, you can duct tape it, put towels over it etc... Once you have a thick bar, short from sand papering it like mad you cannot make it thin anymore.
This will also depend on your priorities. What are you more interested in, grip or back work?