Dartmouth alum here. Spring->summer->fall Dartmouth is one of the best places to train in the country. Endless trails and dirt roads of varying technical level, a 60-mile crushed gravel rail trail and golf course with cart path for the days you need pancake flat, and beautiful weather/environment including a clean river to swim in post run. The only thing it is missing is altitude.
In the winter it is cold and snowy. Like Ithaca, Hanover knows how to handle its roads and one could almost always get outside for a run unless it was actively snowing. On the days where it was too cold to get out comfortably, one could always use a treadmill. Every Ivy gets cold in the winter. Even in the coldest months, the southernmost ivies' (Penn and Princeton) high temps are only 15 or fewer degrees warmer than Hanover on average.
Obviously that's nicer, but also consider the following: Dartmouth is on winter break from mid November to early January so most students aren't even there, Indoor is a very short season, and 3/7 days you are likely working out on the nice (albeit flat) indoor track in Hanover or competing elsewhere.
Faced with the same choice again, I'm heading to Hanover every time.