Hi Aberal.
The direction you decide on is your choice. But (you saw that coming, didn't you?) I'd say it was much harder doing it "in reverse": from Kirrie it's a long long drag up to Glenshee before you even get to the climb of the Cairnwell, which is monstrously steep from the south side. OK, as you say, you get an easy ride down Deeside until you turn right to start climbing the Cairn o'Mount but, tbh, it's not that short from the north side, and while it's a fast descent down to Clatterin' Brig, it's too short too gain much time. But the clincher for me is that it's always felt a long weary slog back from Fettercairn to Kirrie in that direction compared to the lovely downhill finish from Glenshee and down Glenisla.
Waspie, aye, it's not uncommon to get a headwind on the climb of the Cairnwell, but you usually get shelter by the time you start coming down Glenisla.
+1
Going up Glenisla, Devil's Elbow to Cairnwell gives a fairly hefty lot of ascent at the start of the ride - even though you are 'fresh'.
Cairn o' Mount from the north is easier than from the south - it's a bit longer, though less steep
As eck says, Fettercairn to Kirrie is a gentle incline most of the way and is therefore far longer than Kirrie to Fettercairn ! (or was that my imagination
Me ? I'm a wimp - downhill start, downhill finish - this I like
Of course, aberal being a real cyclist from around these parts (there's a clue in his login name