The MM3 will show more data on the screen than the Edge.
The MM3 can have 1000 tunes on it. Most people don’t need more than 1 or 2. 5 tunes with a small difference between each is part of being “generic”, and pretty silly IMO.
I had an MM3 on my 05 and a CTS on my 18... the MM3 probably has 4x the data. I have a CTS Insight on the 18 due to the warranty.
A tune probably won’t add a ton of mileage, but driving a custom tuned truck vs a stock one, or a generic box tune, is night and day. They are so much nicer with a custom tune. You can adjust little things like fan engagement, shifting (some 03’s), throttle sensitivity, grid heater, pilot injection, etc....
You came in asking about EFI Live and are now looking at Edge... two very opposite ends of the tuning sprectrum.