Davester, as others have said, don't be guided by convention. My belief is that the climbs from North to South aren't quite as steep, but there isn't a great deal of difference. Having just said that, if you
do decide to ride South-North, and decide to go past Woolacombe beach (which is simply gorgeous!) you'll be faced with a painful climb that many rather walk. And yes, painful is an accurate way of describing it!
I normally cycle it North-South, but for a different reason: I live in Plymouth, so getting home is no problem. :-)
You mentioned that you'll be riding with some novice cyclists. Can I suggest that you cycle it over a minimum of three days? They'll thank you for it! Also, from Meeth to Hatherleigh you'll be riding on the A386. This is a narrow and busy rural road carrying lots of HGVs, and isn't pleasant at all! I suggest you take the longer, and slightly more hilly diversion that will take you - on a South-North ride - from Hatherleigh, through Sheepwash and on to Petrockstowe, and so avoid the A386. Shortly after Petrockstowe you will join the Tarka Trail for pleasant and smooth traffic-free cycling.
Should you opt to ride along the A386, be warned: on the stretch leading to Meeth there are several uphill, but one in particular is just nasty! To be exact, it is a 13% uphill. Yes, there are worse hills, but this is
not the place to be wobbling along at 3 mph as you strain to make it to the top!
If you do it over three days, I suggest overnighting in Lydford, then
Yarde Orchard (a lovely camp site with a bunk house, yurts and more, smack bang on NCN27).
When leaving Plymouth, you'll follow the Plym Valley Trail to Yelverton. From Yelverton, do follow the NCN 27 route to Tavistock as you'd then get to cycle over the new Gem Bridge - a rather snazzy brand new cyclist & pedestrian viaduct over the Walkham Valley - instead of the route that leads towards Mary Tavy.
Getting out of Plymouth is quite easy: simply follow the directions on my blog:
http://willcycle.blogspot.com/p/plym-valley-drakes-trail.html. I will agree that the short stint of cycling through an industrial area isn't particularly pleasant, though fortunately it isn't very long, either.