VSS vs ESS? Whats the difference

[weedoublewide]

I read that VSS is mechanical and ESS is just electronic? is this correct?
And if so why did the engine I got come with both of the wires being used?

The ECM came out of a 2005 chevy Silverado 2500 HD 4x4? was it for part of the four wheel drive application?

Also, if the statement above is correct, what did everyone use in their swap? an VSS setup or and ESS setup?
Why did you choose to go that route? and what are some good brands of the one you choose to go with,

Thank you,

double