John, the extra length doesn't sound like much nor does the extra weight. These lenses are big and awkward, I think you might find it a bigger deal than you think, just my opinion from dragging mine around for almost three years now.
So how about this idea, wait till they are both in stock and load up and take a road trip to B&H. 4+ hour ride from where you are right? I think it would be a journey well spent.
Second idea, assuming Adorama has a wonderfull return policy like B&H. Buy the 600mm and if it is to heavy and long, send it back and get the 500mm. I would have suggested renting one from lensrental but the return idea is cheaper.
Rick





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