I use an Aera 500 as my main GPS, not having a posh panel one, and the software is much better than the hardware. The screen is just about OK in what the British call bright sunlight, and the screen is just about stabbable in flight, but it's definitely better programmed in advance and not as easy to use in flight as my old Lowrance Airmap, which I miss a lot.
Do you need colour in a backup GPS?
What is garmin's record like for supporting old, old models? Unlikely they will drop aviation GPS completely (which is what did for my Lowrance).