Real quick.. whats the main difference? Do either need ECU splicing?

Thanks

 

Price and the fact that a mechanical gauge opperates by having something physically plugged into it to sample and display. A mech. boost gauge for example has a vacuum hose plugged into the back of it and displays boost based on pressure recieved from the hose. An electrical gauge has the physical item plugged into a sender which then transmits the signal over wires to your gauge.

 

Neither need to splice into your ECU.