You fill out an application before you can even get a price list from Millers. If 50% or more of your income isn't from photography they don't want you as a customer. But knowing people who use millers the only difference is you can call millers but have to email Mpix (last time I asked a question it took 15 minutes for a detailed answer) Millers offers deals on bulk or package prints but thats the only difference.
Adoramapix is good but if you send the same image to each the MPix image is better. better color better contrast better shadow detail.
they cost a little more but I use nobody but MPix now.