Can anyone tell me the difference in kite performance between using regulaer (25m) and shorter (20m) lines, and give a bit of guidance under what conditions / kite sizes you get what difference ? The two kites are 10m North Rebel and 14m North Rhino (both 07). Thanks.

Check www.aaronhadlow.com he mentions it in one of his videos or one of his posts.
Basically shorter lines mean the kite is closer and has less of an arc in which it flies, so moves around the 'window' quicker.
benefits of shorter lines : better response, better de-power, better kiteloops. aaron hadlow uses 20 - 22's depending on what kite he flies (big kites 22's, small kites 20's) and ruben lenten flies 20's. so if ur very into freestyle, probly the way to go.
cons of shorter lines : less power on the light wind days, easier to predict what the kites gonna do as miss-imputs can be changed without the consequences being so bad.

If you're buying from a dealer then see if they've got some sort of demo lines+kites you can try - you'll probly know which you prefer when you try them both

For every 3m of kite line taken off you are effectively reducing the kites power by 1m2 in kite size, this does reduce the power stroke, makes the kite easier & quicker to react to, so having shorter sets of lines for windier days makes alot of sense. The Optimum length for wave riding might be 18-21m, still a good power stroke but close enough to react to better. Most people fly their kites stock out of the bag, usually on 25-28m lines, so do the pros! Or do they?
Effectively short lines does reduce learning curve, giving better control of a kite thats closer to you!
My first power kite from Peter Lynn was on 15m lines, what a huge jump getting onto 20m lines, could hardly fly the thing, 25m even worse....
But you soon get used to it...
Aaron Rhodes had his C Quad on 10m lines, buggying or kitesurfing.
Tim 8)