Well considering fall is much more dryer than spring,I say go with spring,,it don't matter fall or spring as far as apples,,you ain't going to get apples off of it either way for years[if your lucky]..
Fall/spring. I have done both. When i planted my crp project they required a spring planting, but I have had a lot of success with fall plantings. What i will do a lot of the time is figure out in the fall where i will plant and dig up the holes, getting all the stone out of them. I then back fill and add some compost/peat moss. Then when the trees are delivered in the spring its easy to dig the hole back up, and plant the tree. I was able to plant 50 bare root (3-4') trees in an afternoon. Just mark the holes with a bit of flagging so you remember where they are.
I think most dwarf and semi-dwarf varieties offer two things; fewer years before the tree becomes productive and some resistance to fungus/virus problems compared to the standard versions.
Standard sized trees are much harder to keep properly pruned and sprayed if you really want good fruit production.
Dave.
Dave has the right idea, if you want fruit then semi dwarf is the way to go. Picking and pruning are a lot easier.
Spring is the best time to plant new trees but fall is a close second. It's best if the trees have gone dorment before you plant in the fall and they should still be dorment if you plant in the spring.
I recommend Fruit Production for the Home Gardener from the Agricultural Science department at Penn State. In answer to the original question, they say in the spring, as soon as it thaws enough to get a shovel in the ground.