Knowing that your winter temperature can get down to freezing I would be inclined to keep these plants in containers which could be brought indoors during the coldest weather. Saying that, established trees can survive temperatures below freezing so if you got them in the ground and they establish well, they might be ok. You could also protect them on colder days with blankets until the trees are well established. An established tree should be able to withstand short periods at 28.4F. Perhaps you could keep one of the plants as a container plant and the other could be planted, that way if the outdoor one doesn't make it over the winter you haven't lost them both.

Before you plant anything outside, you will want to harden the plants to outdoor conditions. I would pot the plants first to get the roots out of the water that you sprouted them in and after a couple of weeks start hardening them off by taking them outdoors for increasing periods of time over a period of 1-2 weeks. Start them off for an hour or so in a shaded area and bring them back in and the next day increase the amount of time in the shade. Gradually start putting the plants in the sun for increasing periods of time. This will help to prevent transplant shock.

Finally, avocado seedlings have a tendency to become long and spindly so pinching them is a good idea to create a more attractive shape. I found I could be ruthless with the pruning, at one point I cut the whole top of the plant off about 4 or 5 inches from the soil and the plant still came back with no problems....