10" and 650, hands down! I don't know that you get the full split screen modes on the 7" G3X touch, but I'm not certain.
Regardless, you'll be money ahead with a 10" and 650, and the only advantage of the 750 is that you get a slightly better keyboard on it for data entry. All the other 750 added features (like remote audio panel) can now be done thru the G3X Touch.
You'll be interacting with the G3X MFD more than the GTN, even in IFR flight.

(The audio panel shown is going to be upgraded with the new Garmin GMA245, and the AP Panel upgraded to the GMC307 once the plane get's back from the paint shop)