Hi after Ballater you are out of the villages and into totally open country until Tomintoul. Once you get on the part of the A939 called the old military road you will be going into the high peak part of the snow roads, so very exposed. A lot of this part is single track roads with passing places so no real places to park overnight. Probably best to look for somewhere after Tomintoul, this part is bordering forest and I remember some places where you can pull off. Hope this helps.

We had a car, and I did not notice any public bus.

If you have a car, I would just drive around yourself. The tours can get quite expensive. Driving is absolutely fine.

people will have their preferences but I would say each mountain area is different in character, due to the geology, weather patterns (east side is generally drier) and of course the history. Glencoe is more brooding and dramatic and sweeps down to the sea loch at Ballachulish , the cairngorms broader and more spacious and containing wonderful remnants of the ancient Caledonian forest. among many good things in Glencoe you should at least have a beer and food at the Clachaig Inn, where many walkers and climbers gather. a good company to look at if you would like a bespoke program of activities is Wilderness Scotland. Near Newtonmore, cairngorm area, is the excellent Highland Folk Museum, which will give you a real idea of the reality of highland life & culture over the last few centuries. Yes, visit both! Have a great trip! If it rains, that's part of the experience (as are the midges!!!).