32GB should be sufficient for now, but if you have the $ to waste, go with 64GB.
I see 64GB as a more of a down the line upgrade than a 'right now'.
I'd stick with 32GB of memory and go with the D700. It's easy to upgrade memory later when prices fall and 32GB should be sufficient until then.
Going from the D500 to the D700 ($800) is still cheaper than upgrading from 32GB to 64GB memory ($1000).