To answer the question you asked, the highest snow is usually at Aviemore; the largest ski area is Glenshee, and Glenshee is probably more set up for boarding.
Nevis range has a board park but I've never been there.
You can't rely on snow? Yup snow in Scotland is sometimes excellent, sometimes awful. Sometimes there's so much (or there's such a good base that it lasts) you can ski in June :-) Other years you're lucky to ski regularly.
The unasked question: When I was based in Edinburgh, Glenshee made loads of sense. When based south of Reading, I decided skiing in Scotland made little sense, because costs were comparable with going to France, and conditions there were so much more reliable. Usually. Would it really be significantly cheaper to head North?
The Bell brothers and many others trained at Hillend - when young. When they got good enough, they skied in the Andes on New Zealand in Summer and the Alps and America in winter - in the brief days when the British squad had money. They'd pop back to say hello to their old pals. There are a number of ski racing schools for Brit juniors; they're based in France for a reason.
Yes, as their name implies, the Sierra Nevadas get snow. Their piste map lists lots of runs but the bottom ones are often closed, so it doesn't really quite live up to the potential on the map. (Same applies to quite a few Alpine resorts as well, though)
And, ahem, though I live in Edinburgh I wouldn't spend a week skiing there :-)
This is a reasonably fair but relentlessly upbeat comparison: