Here's my view:
Volunteers aren't that good as if they volunteer to be one, they won't work as hard as they are getting nothing in return except a new group. But if you find volunteers that have already been working hard on your site, they have a interest in it, therefore their work will be at 100%, trying to make your site better.
Where as paid ones, they know they are getting paid for their work, they will try to work as hard as possible in case the owner thinks they're not working hard enough and losing their moderator roles. However, the paid ones may not have a good interest with the forum you have therefore the work he does may not be at 100%.