I would use the A9 and A96 as the quickest and easiest and most fuel efficient route. I would not use the B roads around there as these are slow, narrow and full of bends although not sure which you mean. If you want to take your time and enjoy the scenery you could take the A95 just North of Aviemore and then the A939 to Forres. That route is quite easily passable, but far slower with lots of bends and despite being shorter will, all other things being equal, take longer.