We were there 7/3 and 7/4 this past year. Watched the fireworks both nights from the Poly then I went to the MK after we put the kids to bed (both nights have some later hours).
I can’t prove this obviously, but I found 7/3 “felt” more crowded at the Poly and MK. I think some of it is that many people choose MK for 7/3 and Epcot for 7/4 since Epcot only does one night of fireworks.
I also think some of it, at least this past year, was 7/3 was a Monday and 7/4 was a Tuesday. I think with a lot of people having 7/3 off, it became a long weekend for many. With some people going back to work 7/5, 7/4 itself had less of a local draw.
Again, obviously just a generalization, but that’s how it ‘felt’ in my travels. Had a great time, we really enjoyed being there for the holiday!