You must be desperate for sig exposure: I haven't seen anyone ask that question for years and you surely know the answer
The most popular reply used to be that for people with very stable usage, there
may be a small advantage to 95%. Those who have a lot of major peaks in their traffic (which includes most TGP's, free sites and submitters' domains) are better off on a per gig basis. That of course assumes otherwise equal pricing.
I never delved into the truth of otherwise of the above. I prefer per gig because otherwise I hadn't a clue until I got the invoice what I was going to have to pay
