Dwarf apple trees: container or ground?

[Soliloquy]

I've seen plenty of advice either way, but I'm hoping to hear opinions from people who have actually grown dwarf apple trees. Can I grow them in a container (if so, what size?) or must they be planted in the ground? The actual trees in question are about a year and a half old, on Bud 9 rootstock, and I'm in zone 6 (New York City).