I had a Surefire 500A light. Once I saw and used the 900 series light, I sold the 500A.
The reason is that you don't use the light very much. When you need it, it is essential, but you don't use it all that often. But, with the 500 series light, you have to carry it around attached to your carbine all the time. It increases the weight of the carbine, and it changes the balance of the carbine. Lastly, there is always the chance that you will break the light during everyday usage. The 900 series light gives you everything the 500 series light gives you, and much more. The main thing is that with the throw of a lever, the light is off yoyr carbine. The weight is gone. If you need it, it can be re-installed in seconds. Then there is the matter of rails. I own something like 11 AR15 carbines. Most have regular handguards, two have rail systems. The rail systems are a vast improvement, not having them is a significant step backwards. With the 500 series light, you obviously can't use any other handguard. With the 900 series light, you can use any handguard out there. Another great thing about the 900 series light is that because it can be easliy removed from the weapon, I could use the 900 series light on any one of my AR15s with almost no effort. Just throw a lever and put it on a different rifle. This can be done to some extent with the 500 series lights, but it isn't nearly as easy and it could only be done with another carbine that uses the same size handguards. For example, if you get a carbine length 500 series light, you couldn't use it on your 20" rifle. But with the 900 series light, you can. In fact you could install the 900 series light on any gun you own that has a picitanny rail.
Versitility is the name of the game. Why strap yourself down if you don't have to ?