Either way is good.
1. If you're quite slow and going S to N, might get benighted at the top of the pass, which would be serious. N to S means quite a bit at the end is on easy and fairly low-down paths, better for getting benighted. If you've a driver and are finishing by Chalamain Gap to the high car park at Sugar Bowl, then the Ch Gap would be really really nasty in the dark. Really!
2. Otherwise, it depends on transport. If using public transport, travel in to Braemar, as buses from Aberdeen are fairly well spaced; then it's dead easy to get back from Aviemore by bus/train at any reasonable start time.(There's a not useful postbus to Linn of Dee around midday.) You could overnight at Inverie YH near Linn of Dee.
3. Other things being equal, do it S to N as the finish through Rothiemurchus Forest to Aviemore is really good, especially by moonlight (divert to Loch an Eilein).
4. If starting from Braemar rather than Linn of Dee, go through Morrone Birkwood (lovely) and cross Victoria Bridge; avoid almost all tarmac.