I guess the answer depends on your goal. If it's resale value, my completely WAG is Garmin, simply because of market penetration and familiarity.
If that's not a concern, the answer is really "which do you like better?" I've flown with both; the Garmins in club aircraft; the Avidyne in a friend's. My take is they both do their jobs well, but if I were in the market, I "like" the Garmin better. Why? For no reason other than, despite obvious changes in the interface, having flown with Garmin handhelds, the 400/500 series and some with the G1000, I've generally found the underlying Garmin logic and flow, for better or for worse, consistent. That, I think makes the transition to the GTN relatively simple. OTOH, one of the reason my friend chose his IFD was, he was a user and lover of the 480 and found the Avidyne's logic to be more similar than the Garmin.