The type of technology is "inertia" that I'm assumeing is in the Nike Speed Distance Monitor you talking about. Its actually a pretty complicated system and is only accurate up to 97% when you're running are even terrian,and at a steady and consistent pace. I've heard its less accurate on downhills because of slipage and on wet ground. The Polar S625x I believe works off the same technology and some have claimed to have close to 99.5% accuracy. But mind you these people aviod varying terrians, puddles of water, and try to keep the most consistent pace possible in order to achieve that 99.5% accuracy. I myself, a typical runner, have no time to worry about avoiding such variables. Running is about unpredictable variables (ie. hills, rocks, puddles, mud, streets to dirt). That why I believe GPS is much better.