Full disclosure I’ve never been to either so I’m going on what I’ve heard, but Sugarloaf on average has better snow conditions when the Northeast is having generally good conditions. More pros of sugarloaf (relative to Sunday River) is that it has above tree line skiing, large expert “side country” (bracket basin, burnt mountain, eastern territory), more expert terrain, has longer spring skiing season, and overall larger mountain. Pros of Sunday River relative to Sugarloaf is that it’s an easier drive (unless you live in Northern ME), has shorter run outs after trails, more modern lifts and amenities, and better percent open (snowfields and “side country” at Sugarloaf can often be closed when the Northeast in general is having bad conditions). Ideally check the snow conditions before going because this March is supposed to be warmer than average, which can close a lot of terrain and might not make the extra 2 to 3 hour drive to Sugarloaf worth it. I don’t know much about crowds but I’d think Sunday River would be more crowded because it’s closer to Boston and NY.