John, Ive used PM on a couple occasions (maybe 10 years ago) and their work is very good. Plus John Davis is really a nice guy, to boot! He gave me a tour of their facility and it was great to see the various posters that were all being worked on.
Their prices are higher, tho, compared to some others (Ive never used Studio C), so that's also something to keep in mind.
And Ive only ever heard and read good things about Studio C, too. I opted for PM because I could drop off and pick up my posters. Studio C would have been a much longer drive.