I am looking at putting in oil pressure and water temperature gauges and see that there is choice between mechanical and electrical ones. Which seem to be better?

Personally, I tend to lean to electric.
Much easier to route a couple of wires vs a thermocouple and oil tube.
Also--an oil tube can crack inside the car---mess.

Sending units for the electric gages NEED to be grounded VERY well in order for them to work. Sealant on the threads of the sending units tend to prevent them from grounding.

I seal 'em anyway--just a little--and tighten the units securely. Then I get out my trusty meter, and make sure that there is no resistance between the outside of the sender and the manifold or block (meter reads '000')

I have Dakota Digital in the 59-----way happy with 'em