Wiring electrical stuff is easy once you understand the basics. It's running wire through the car and making it look neat and factory is a PITA to me. I just got done testing my elec water gauge in a cup of boiling water using a meat thermometer for comparison.
Elec gauge:
Make sure the 12v source is switched or it will work all the time, and for the best accuracy I believe you want to ground it to the engine (you can ground the light bulb in the car.)
The mechanical gauges are a little quicker to respond to changes, cheaper and are full sweep like 5L5 said. I have a mech water temp on my cowl temporarily and I love the way it works, but its gotta go. The sending unit is in my thermo housing and thats where my fan switch is about to go for my new fan, and I'm tired of having a gauge on my cowl.
If you have the extra 3/8'' port on the back of your lower intake, (unlike me) get the mech gauge. I can't get the capillary tubing in the cabin because all my ports are at the front of the engine.
You know I'm this close to buying pillar pods, but I'm afraid the said morons trying to race me at night. The cowl gauge gets questioned a lot too by regular folk.