Adorama vs Mpix
Sorry if I'm "resurrecting" an old thread, but in case anybody still cares, I've used both & strongly prefer AdoramaPix for one reason--they offer "full frame" prints, whereas last time I checked, Mpix doesn't.
This is huge to me, as I detest any cropping whatsoever. I compose my images a certain way at capture time for a reason. I then continue to see the same details on-screen. I do NOT want details lost due to mismatches between the aspect ratio of the given paper size vs the original image. Yes Mpix will do 8x12 or 12x18 prints, but good luck finding frames that size that aren't crazy expensive. AdoramaPix lets you choose "full frame" as an option whereas Mpix doesn't. That alone for me puts AdoramaPix ahead of Mpix.