Look at the specs...
Krait:
1600 dpi infrared sensor
Up to 40 inches per second and 15g of acceleration
3 buttons
Approximate size: 120mm (length) x 60.5mm (width) x 37mm (height)
Diamondback 3G:
1800 dpi 3G infrared sensor
60?120 inches per second and 15g of acceleration
7 buttons
Approximate size: 128mm (length) x 70mm (width) x 42.5mm (height)
Copperhead:
2000 dpi laser sensor
Up to 45 inches per second and 20g of acceleration
7 buttons
Approximate size: 130mm (length) x 66mm (width) x 41mm (height)
32KB Razer Synapse? onboard memory (stores driver profiles on the mouse)
Obviously the Krait is the smaller mouse of the 3, and with only 3 buttons it's more of a no-frills option. Razer claims it targets RTSers and MMORPGers due to its 1200 actions per minute or whatever BS (they just needed a small/lower end mouse to compete with the Logitech MX3xx/G3 mice).
The Diamondback 3G has the most robust sensor of the 3, able to continue accurate tracking when used at very high speeds (the best option if you're low sensitivity gamer and make very fast/long swiping motions with your mouse). It probably has the best shape out of the 3, the original Diamondback was/is pretty popular (despite their similar looks, the 3 mice are actually very differently shaped).
The Copperhead is good for higher sensitivity players and its unique advantage would be its on board memory allows you to setup driver profiles and store them on the mouse allowing you to uninstall the drivers and use the mouse as if you had drivers installed, you can move the mouse to another computer and the settings for the 5 profiles will remain the way you set them up and the mouse will function the same way across different computers - with or without drivers. It was the first mouse to incorporate this and it is becoming a new standard for high end mice (Logitech put memory on their new G9)
I would say the Diamondback 3G would probably be the best mouse out of those 3 options when considering all the strengths/weakness of all the mice as well as the price.