- This is a good upgrade
If you have a gun that doesn't have as high an FPS as you want? The M110 spring is the upgrade for you. Sure, it doesn't give as much power as the M120's, but it doesn't put your gearbox at risk.
Oh, wait, you didn't know that? The action of the Spring being streched and released time and time again can put massive strains on your gearbox, to the extent of actually shattering the metal. Unless you have a reinforced gearbox and other upgraded internals, you want to stay away from the M120.
Buy this M110. It is not held under the same tension as the M120, and does not pose a threat to your AEG. Chances are, if you need to upgrade your gun to get it to fire 340 fps, your gun probably isn't capable of hosting and M120. This spring however will give you your power boost and keep your gun safe.
There is also reason for people with higher power guns to get this. By downgrading your spring from whatever it was to this, even if it means losing 30-40 fps, can be a good choice. Your ROF will increase and your gearbox will last longer. Your motor won't burn out quite as fast, your gears won't strip, your piston won't shatter quite so soon; it's a pretty good chance that you could increase your gun's lifetime by a manner of years.
So you can see, buying and installing this spring is a good choice for anyone. The M120 might appeal to the power junkies, but rest assured, when their guns stop working months after they upgrade them, you can laugh in their faces as you shoot them with your fully functional AEG. Spending $16 and 30fps for this spring versus $400 every few months for a new AEG; I think we can all see what the better deal is.