Aye, I think you have it right. The road to the north over Folly Top is one big hill (then another littler one into Middleton) and usually very exposed to the wind, the southern road through Lartington, Mickleton etc is
more benign a bit less hilly and much less exposed, though it's still a series of ups and downs as you cross all the rivers and becks flowing into the Tees. The first climb out of Startforth is a bit of a pig, too.
They're both b-roads, but the northern one is busier (as it's faster for cars - it only passes through one village rather than half a dozen).
The south road is the one to take. Mind you, according to Nigel Hall, it is in fact 200 yards longer than the northern route