I'd go with Proficient in a heartbeat, over Muellers. Aren't they mostly just a catalog? I'm only guessing here, but they probably subcontract work like that out.
Personally, I sent one cue to JW Petree (an AZB member) to refinish, and was so happy with the work that he's now got another one to work on. His work is great, the turnaround is quick, and the price is very reasonable.
Cory
UPDATE: I just read read Ratcues' post above. My concern with Mueller's was that you wouldn't know who's doing the work -- they could send it out to anyone. But it appears that there is a single person with a lot of experience that does their work. Still, it seems like they should mention you by name on their cue repair page.