My guess is this is probably a firmware update, and the mouse itself is physically the same, resulting in shit performance. If this is true, I would probably be pissed if i got one of the 1800 DPIs and it acted like crap. Don't ask me why Logitech felt like it had to squeeze 200 more dpi out of hardware that was engineered for 1600. Idiots.
I am really not sure what point you are tying to make, but its not a firmware upgrade, its a hardware one. The appearance did not change, just the optical sensor was upgraded.
Guys iam still confused between these two mx 518 and microsoft sidwindor x3 as sidwindor is laser mouse and mx 518 is opltical will that make a differnce
You are trying too hard nikhil. The difference between optical and laser is very subtle. In an optical mouse you will observe a bright red light being emitted from the underside of the body. Now, while playing a game like CS late nights it might be a distraction for you. Laser mouse will emit no visible light, there is nothing exciting on the underside to see so you will concentrate on playing competitively rather than looking at the mouse.
If you are really attracted to bright lights, I would not recommend an optical mouse cos you get easily distracted and will be pwned by a noob with a ball mouse.
On a serious note, it all depends on your budget. You said Rs 2000 and all I can see is the MX 518 available in India. I don't see X3 being sold there. All I see are X5's and X8's which are very expensive for your budget. Anyway, as I said if bright red lights are not suitable for you, go for the x3, if not go for the MX 518.
My personal choice would be the MX 518.