Peter’s photoshop has inspired this post. We will not debate the wisdom up installing a GTN into a Rallye and upgrading it to IFR…
Assuming a madman does so, does it make sense to install a GTN750 for the double duty (as opposed to a GTN650), of being an MFD as well as flying approaches? I know IPads let you know where you are, but I mistrust wires in the cabin, batteries and summer temperatures.
FWIW I’m getting a top overhaul done on my plane (cylinder corrosion), and simply don’t want to deal with the brain damage of getting another POS aircraft into proper shape, a process that seems to take about two years. I would rather fly a well-sorted slow plane than dream about flying an absolutely brilliant aircraft that is devouring money in the shop.
The user interface on the GTN750 is superior to the GTN650 and worth the difference in price IMHO. There is much more margin available in the GTN750 and a competitive dealer can earn more from an installation on a GTN750 than a GTN650. So I advise pilots in the US to negotiate the best competitive deal on a 650 and then tell the dealer he has my business if he accepts the same profit on a 750 install.
Get yourself a decent iPad mount with integrated cooling
Spend the rest on flying
I would definitely get the 750. It was clearly the one that was designed, then its excellent design was put in a vice and all the life squeezed out of it.
FWIW I put a 650 in my a/c, partly due lack of space, but it seems to do everything I require and displays approaches adequately. The Garmin display of airspace is very poor (UK)and SD does a much better job of airspace depiction.
Try the simulator and see what you think.
@Robin_253
That advice is so sensible as to be irrelevant… 
Just as a different data point – the Avidyne IFD540 has the same functionality as the 750, but is much smaller in size. Somewhere between the 650 and 750. That was one of the reasons I chose mine, as it took up a lot less space than a 750.
With an iPad Mini “connected” to a GTN650 by FlightStream 510 you will have a decent sized moving map, convenient flight plan entry/modification, and no wires. If either the iPad or the GTN650 goes t/u, the other will continue to operate independently.
However, airspace depiction for VFR on Garmin Pilot is still the industry’s worst, so you really need a smartphone running EasyVFR or SD as well.
Don’t know the price difference (company pays ) but the 750 is definitely easier to use than the 650.
The 650 feels like “750 functionality compressed onto a small screen”. There is scrolling with onscreen up/down buttons involved etc. In a way the display is more like a 430 series.
