Optos tend to vary pretty badly in their current transfer ratio from one part to the next, some of them by 4:1 or so, and are linear in current, not in voltage. Getting the system accurate will be a pain, and will most likely require you to supply power to the isolated side to be able to have some active circuitry over there to help.
That being the case, as long as you're going to have to supply power to the isolated side, you might as well just move the A/D over there as well. Digital signals are much more forgiving of trying to push them across an isolation boundary; a poorly linearized 1 or 0 is still a 1 or 0.