A pure mileage tax would weigh as heavily on somone driving a 3-cylinder Geo Metro as someone driving a Hummer. Doesn't seem fair to me. Yes, it is essentially a sprawl tax, but it gets rid of the environmental incentive of a gas tax.
I think a mix of the two is appropriate, though there are tons of questions tied up with a mileage tax. To wit: what about older vehicles? Who has access to my GPS telementry? Will I get a speeding ticket based on GPS readings? What's to keep me from disabling my GPS transponder entirely? Probably more that don't occur off the cuff.