Vet all the way. Dentist can NOT give the calming shots (at least in MD) and most horses (in my experience) really need one to behave during the process. So you either have to have very quiet horse, which is OK about floating, or coordinate vet and dentist appointments, which is a real pain. So I just went with the vet, who's second specialty is dentistry (he LOVES doing it and does lots of horses).
Also no offense to anyone, but I trust vet (with 8+ years of education + who knows how many years of experience in both - veterinary and dentistry) much much more than some guy with 6 months of some classes on something.