Well, I own neither. But if the intended function is as a backup EFIS in IFR, then I'd recommend buying what you don't own. e.g., G5 only if you do not have a Garmin panel; Mini only if you do not have a GRT panel. And do not interconnect it to anything. You don't want your backup connected to a failed device.
Just my two cents.
The G5 is built with features and design intent to prevent this failure scenario from rendering the G5 useless. Dissimilar design, dissimilar hardware, dissimilar software (even a dissimilar compiler from my understanding), essential features only, etc.
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I'm aware of the Garmin claim, and I applaud them for making the effort. But IMHO human error (software) remains the big unknown, and personally I think different manufacturers is a bit better route - unless the Garmin engineers never talk to each other at the water cooler! I won't scream or rant if someone chooses a G5 for a Garmin panel; but I do recommend NO interconnects. Just too hard to predict what strange signals or voltages might come down the line in the event of some unpredicted failure.
In my old panel, I had Garmin for the GPS/NAV & Com, GRT for the EFIS and Trutrak for the Autopilot. There were some functions that never quite worked right between the EFIS & the GPS/NAV. I had to manual switch the AP between the EFIS & the GPS/NAV depending on what I wanted to do.
r.
I am very curious about when you had to use that switch. My set up is similar (GRT HX, G420W,SL30, Trio) and I never have used that switch. As far as I'm concerned its only use is if the Hx dies.
I had a GNS 480 and Horizon WS.
The GTN also does a much better job of seamlessly switching from enroute GPS to ILS/VOR approaches. I always found that to be quite cumbersome in my old set-up and I didn't do it often enough to remember all the buttonology.