konkers - Choosing between the 15 and 17m really depends on what wind range you get where you live. If you average between 12 and 17 knots, and you are 70 kgs, I would go for the 17m, as I am now 52 kgs and on a 16m Naish am perfectly powered between 12 and 15 knots, and slightly overpowered in 15 and 16 knots.
However, if you average 16-23 knots, I would go for the 15m definately. I am powered on my 12m Naish in 16-18 knots easily, or just as well on my 10m Cab ele in 16-23 knots.
Anything above a maximum of 17/19 knots and gusting even higher, the 17m would be un-useable. The 15m is nearly a good all 'round weather kite from 16 knots onwards

That's what I've found at least