I use both and the comment above pretty much said it all. Millers provides at the end of the year a earnings statement, while Mpix has a check box at each upload that asks you if this is for resale.
I can see no difference in the photographs but the phone support from Millers is truely missed if you deal with Mpix.
I was forced to us Mpix because Millers does not ship to an APO, that was a bummer.
Each month Mpix is making efforts to come online by offering Mpix users as many options as they offer the users of Millers. So I would go with Mpix.
Now granted you do not get the bulk order discounts you get Millers if you use Mpix, but you win some you use some, unless you order alot of the exact same image.