I compared the various Garmin products at Pacific Coast Avionics a couple days ago. This was indoors, but I have plenty of experience with 696s and 510s in RVs, so I think a comparison is valid.
The 79X screens were not as bright as the 69X. Maybe color saturation is a better metric. The 5XX Nuvi boxes were clearly fainter. BTW, no other manufacturers boxes were as clear and bright at the 79X. The 69X is the top pick for legibility. I wish I paid more attention to slant angle, but I think the touch screens faded quicker with offset.
Airborne, the 696 remains very legible in sunlight. The 510 is like looking through a fog. So I would expect the 79X to be somewhere between. Operation of the the "Home" button devices hardly requires looking in the manual. The 69Xs are much more difficult.
The deal killer for me is touch screen. Maybe in a flight level machine, but a bouncing RV guarantees often missing the mark when you poke the screen.
This doesn't require moderate to severe (where you you want to scrunch down in the seat for more canopy clearance) to be annoying. Buttons are superior in this alone.
Only problem is: Garmin will probably drop the 69Xs within a year, then support will fade.
John Siebold