In my opinion (especially with freezing temperatures), in the world of regular AG pools, I would go with a round pool of larger size. I may be biased since I have one, but here's my reasons:
1. If you have kids, you can still do laps without them getting in your way, heck, they can even play volley ball while you swim laps through the center.
2. Less problems with regards to stress and wall shifting over an oval, especially in climates where frost heave occurs.
3. Easy to build in comparison to a buttress or buttress free pool.
4. Round pool seems to give good flow from the return all the way around, I see my one neighbours pool (oval) getting less circulation at the far end opposite the return, since his stairs are there, it gives him issues with algae regularly. probably due to non circultaing water.