Since some things haven't been said here, or has he been answered yet, I'll toss in some stuff.
The number you see for spring strength refers to the velocity will shoot in a standard test set up at the factory. No idea what the set up is, but the number you see is in meters per second, so a 100 should be around 330fps, a 110 should be around 360fps, a 120 should be around 390fps. Different manufacturers use different test set ups though (I say test set up because I can't see them using a gun to test with), so you get different velocities out of the same rating spring. Aka: A Prometheus 110 spring might net you 360fps, a Modify 110 might get you 390fps, a Madbull 110 might net you 425fps (these are what I've seen in my gun tech days).............. I'd recommend also the Modify springs, they are the most predictable springs as far as ratings go, get the 100 spring, it should sit you around 370fps). Modify also makes some pretty impressive parts too, exceptionally good for the price you pay. Madbull, I haven't seen too many good things come out of that company.