Having used both services previously myself, I’ve always found Photobox to be the better of the two in terms of the quality of prints. They are also the guys behind the Carphone Warehouse digital prints service.
I’ll let you in on a little secret as well. For those of you who are already signed up with Photobox, don’t fret as the registration is done on the persons email address NOT postal address. I think you know what I’m implying 😉