I know the Omori Boost Gauge matches our interiors and I've already decided on that product, however... I'm curious which to get... Mechanical or Electrical?

Electrical costs 100 bucks more... but is there any advantage to it?

I guess I'm just asking for a rundown... Mechanical vs Electrical... Advantages, Disadvantages, and why would one be chosen over the other???

Thanks in advance!

Both work fine, the response of the electrical gauges may be snappier but accuracy should be equal. This is due to the restrictor/damper used in line with the mechanical gauge to prevent the needle from oscillating or buzzing. You can try to use the gauge without the restrictor to make it more responsive at the risk of having the needle vibrate, or you can try different size restrictors instead of the one provided. But I don't think it's a big deal.
Installation of the mechanical gauge is also easier, just the one hose to connect via "T" fitting in the engine compartment and then run into the car, and the illumination wiring to the gauge inside. The electrical gauge has a separate sensor which has to be wired in the engine compartment.
Also note that the electrical gauges have a single ground connection for both signal and illumination. Since the Subaru dash lighting uses a variable negative system for dimming rather than the usual variable positive, you will NOT be able to have dimming on the gauge without an external circuit to reverse the illumination polarity. The mechanical gauge has only the two wires for illumination so you can connect it directly to the factory dimmer circuit.
I have the 45mm Omori mechanical gauge w/full-color illumination and I'm happy with it. Installed it about a month ago in the factory 3-gauge pod, in place of the OEM Lamco gauge.