Speaking to prospective customers I have found that many want fixed pricing. It's easier for them to get budget for $x than to get budget for $y and then have

Speaking to prospective customers I have found that many want fixed pricing. It's easier for them to get budget for $x than to get budget for $y and then have to ask for $z because they used more data than expected. So it really depends on the audience - if you have both audiences then you need to have two plans - tiered and pay as you go.