If you are level 6/7 skiers you will be better served by taking your lesson at Snowmass. It has a greater abundance of terrain. That variety of terrain will give your instructor more options to help you progress.
If you are worried that there aren't the steeps at Snowmass that there are at Highlands, let me assure you that there is terrain at Snowmass that is every bit, if not more, steep than that at Highlands. It's just that Highlands doesn't have the variety of intermediate and low expert terrain that may be useful to help you progress. Not only that, but Snowmass is a much larger mountain and an instructor can help you to see and explore more of the terrain.
As for an instructor, I'd highly recommend Mark Bagby. He's been my coach for the past several years and is a recent stellar addition to the ski school at Snowmass.