Depends on the application - you should be able to think of a few off the top of your head.
i.e. what are the main differences between belts and gears?
What sort of things you can think up yourself tells us how to best respond to your question.
Some quick things to get you thinking are:
-Are there any fluids going to be around the area you want to transfer power at?
-How far away do you need to transfer the power?
-Is weight a consideration?
-Do you have a place to mount all the gears?
-What kind of ratio do you need?
-Will you have access to the part to change it when it fails?
The answers to some of those questions may help drive you in a direction.
How much slip can you tolerate under load?
If the driven machine seizes, what will break or slip, what consequences?
Dry dusty environment, what are the risks of fire?
Do you class a roller chain as a belt or a gear?