Before all, both labs provide top tier printing services. Their bar is remarkably higher than snapfish / shutterfly / walgreens, cvs, target, etc.
I have been a adoramapix customer for three years until a friend recommended mpix to me. I had a set of pictures, taken by Kodachrome using a Contax T2, developed and scanned by Dwayne’s Photo. The resolution suffices 300dpi. I had adoramapix print one set and mpix print another set using the same original jpeg files embedded sRGB color space. (The extra set was for my parents.)
I chose metallic paper from both labs (no lustre coating from mpix). For a 4×6 print, it cost 29c at adoramapix and 59c at mpix. Mpix roughly double the price.
The packaging of mpix is slightly better but both suffices protection purpose.
Both of them use Kodak Professional Endura paper, adoramapix has a more shining finishing in its surfaces. The color are almost the same compared with my calibrated monitor if viewing by naked eyes.
For a long time I was not happy with the details of adarama prints – the resolutions worse than my Canon MP980 printer on Canon Photo Plus paper. Mpix is doing better but not much – still worse than my home printer. However, both of them are visually more attractive than my home printings, especially viewing from 1+ feet apart.
Talking about the “edges” (object borders) in the prints, mpix is slightly more crispy. Mpix offers more stereo human face on portraits than adorama. These can be identified by naked eyes. Using a Pantex 5.5x Loupe, I could see finer edges on mpix prints with about 1.5x resolution than adorama. There are more noise on adorama prints (white dot on dark objects borders are vice versa).
Overall, mpix is about 20-25% better than adorama. But I do not think it worth double the price. For photo enthusiasts, AdaramaPix is good enough.