authorized dealer doesn't matter, authorized service center matters though they usually go hand in hand. The point is that you need to find a shop that services regularly with guys that do nothing but service regulators on a daily basis for them to be good. I have never been to DRiS's service center, but I would send my regs there. I have been to Air Tech many times and have had my regs serviced there many years ago but I still pop in to say hi every once in a while, but I will say that they are probably the only shop in the country I would recommend anyone to send their Poseidons because I know for a fact that they service them regularly, carry parts all the time, and know what they're doing. Aqualungs are pretty standard, any good tech can tune them in their sleep, just find a good one. If you want to use Divers Direct that's fine, don't know anything about them, but I would choose from one of the two shops I just mentioned and you'll be quite fine.
DRiS is $10/stage cheaper if they were serviced within the last year, $5 cheaper if not than Air Tech so I'd go there based on price alone.