Expect the Sensei I have used/experience all of the mouse you listed so I can give you some personal usage feedback among the mouse
The choice is up to your preference, type and usage.

Logitech G9X
> Claw Type Mouse
> Matte Plastic Body
> Adjustable Weight
> Interchangeable Grip (Soft Rubberized Grip / Durable Texture Grip)
> Smallest among the mouse you listed but tend to be a little heavy.
> 5 Onboard Memory Profile(each different Profile would able to customize various of colors you want)
> Smooth Scroll Option

Logitech G700
> Palm Type Mouse
> Texture Grip on Side Button Area, Matte Plastic on Top Surface
> 5 Onboard Memory Profile(unlike G9, Profile Change was limited to Yellow Color and it selected in LED Up & Down)
> Wireless/Wired Type (Comes with USB Dongle for Wireless Connect and Micro USB Cable for Wired Operation)
> Most Button among the List
> Smooth Scroll Option

Microsoft Sidewinder X8
> Palm Type Mouse
> Full Matte Plastic Body
> Full-Time Wireless Type (using Magnet for attach/detach towards the round charger/connection dock)
> Mini LCD Display (Able to show DPI Value, Icon of Recording Macro)
> On-The-Fly Macro Recording

Razer Death Adder
> Claw/Palm Type Mouse
> Rubber Grip Top, Glossy Plastic on Side
> Cheapest among the list
> Lightest among the list

Steel Series Sensei
> Palm Type
> Metallic Finish Top
> Onboard Custom Profile Settings by using the LCD at the bottom
> 5 Onboard Memory + Awesome Multi-Color Changes on Scroll Wheel and the Back of The Logo in different profile
> Customize-able Logo on the bottom

This post has been edited by wingster: Jun 12 2012, 01:22 AM

Its awkward at first, but once you get used to it (take me about 3 weeks time since I brought it last month) it work pretty well expect FPS which I still prefer the G9 or G700. Also I recommended using smooth less friction mousepad like for example the Razer Goliathus Speed Edition, before that I was using on my QCK which is quite hard to navigate.

Overall the G600 has far ahead better software (same goes to G300 and G400) than the previous logitech gaming mice including the G700 but ergonomics and trying to lifting the mice is quite unnatural/uncomfortable.

This post has been edited by wingster: Aug 13 2012, 12:02 PM

Logitech G9x is a very solid mouse but despite it size it was heavy and suit for claw type user.