We always assume the Garmin with the most climbing and distance is most correct.

OK, really, I have the same issue. Same ride, but the 305 shows more climbing than the 500 when uploaded to Strava.

I think it's a philosophical issue. What is climbing? Is every tiny bump, pebble, and compression of your tire part of your elevation gain? Or should it be smoothed? If smoothed, how much?

It's like asking how long the coast of England is. It's all about fractal paradox. Really.