Welcome!
Mechanical uses a vacuum line directly from the source, and to me is the better choice. Electrical uses a line to the sender unit, which interprits it and send the information to the gauge to display. However, if there are any electrical problems, faulty wiring, or guage problems from bad shipping, you could get totally wrong readings. I personally run an electric Prosport boost gauge and it saws I'm running 2psi when I run 7-8 :/
52mm are the most common. I'm not a MS6 owner, but I'm pretty sure that's what everyone uses. The guages that match your interior are the Prosport Amber/White series. They are fairly cheap. The only thing better imo is the Defi gauges, but it's close to impossible to find a good, working gauge.