I have the exact same grinders and use them exclusively for espresso. I would say that the DF64 produces brighter flavors and the J-Max a thicker body. But, as I use the J-Max as part of my travel kit with a Flair Pro 2, the difference might also be attributed to the narrower and deeper puck of the Flair compared to a traditional 58mm basket. I'd also tend to attribute the differences to blade geometry rather than size.
I've heard that larger burrs produce less heat and help retain more flavors. I don't know it there is a serious scientific study confirming it.