Hi Malky. I have driven them both extensively also. Yes the mileage is almost the same. The loch ness section can slow it down a bit in the height of summer I suppose but still my choice for speed. I used to drive up to Kyle for work and I know the road to Skye like the back of my hand.
I would sometimes drive back via Achnasheen, Garve if I was in no hurry to get back ie skiving
If both roads are empty like very early morning I would say the Loch Ness route is at least 15 mins quicker, and that's with out driving like a maniac.
Haven't come back via Strathcarron for a while now but will probably give it a go and see how my times differ as you have got me thinking. It certainly feels longer.
Where I live in Inverness the new Ness bridge is an added bonus for me whereas the traffic over the Kessock Bridge and associated roundabouts can slow me right down at certain times.
Cheers, I will let you know how I get on.