We live in Southern NH and chose to do a "portable spa" (above ground) with to go with our in ground pool. We did so so we could use it year round (as you stated you wanted to) and we do.

You can use an in-ground spa year round, but you have to have the heat going and pump running to keep it defrosted. Most people suggest against an in-ground hot tub with an in ground pool if you want to use it year-round, because most of the time the plumbing is connected...