Well I have used the G9 a couple of times and the ergonomics is killing my wrist. I have a few other friends who had the exact same experience. And when it comes to sensor placement, balance, and glide the G9 is not nearly as good as the SS, the DeathAdder or the older MX518 which is the last great gaming mouse from Logitech in my opinion.
I find the weight system useless you should always go for a mouse that is in balance to begin with not one where you can even it out by adding lumps of metal. A mouse should be balanced and light when of the mat.
There are a lot of factors to consider, but ultimately you should go with what you are comfortable with, and fell provide you with a good "tool" for games or work. The G9 does not cut it for me as I feel there are too many obvious errors in the design.
The thing that bothers me the most is that the Logitech designers should know all the pitfalls in mice design by now, but they have not been able to come up with a halfdecent design since the MX518 which really combined it all for me, a great sensor that can track movement precisely and at great speeds with good ergonomics. Plus it had buttons that were great to use and were placed correctly. The G5 missed a button and had a way inferior laser sensor, the G9 has a better sensor (still not as good for low and medium sensitivity gaming as that of the MX518), but it has bad ergonomics and a lot of useless features.