Try this - go to the Apple website. Click on "iPhone" -> "Learn More" -> "Tech Specs", and scroll to "Display". The two models are listed side-by-side *by default*. Could the differences in the displays be made with any more plain language? Hell, the resolutions and PPI haven't changed since the 6 and 6 plus years ago, it's not like the information hasn't been out there for some considerable time.
The decision you made on your 6S plus purchase, if you considered the screen at all, was based on the same screen resolution, size and therefore PPI. Just as the 6S would have been a downgrade from the 6S plus, the same follows for the 7 plus and 7. Just because the colour gamut improved doesn't mean anything else did, for that you have to check the specs - especially if it's going to affect your livelihood or hobbies.
"1334-by-750-pixel resolution at 326 ppi" vs "1920-by-1080-pixel resolution at 401 ppi". Seems pretty clear to me.
Also, I'm not sure why a photographer would buy the non-plus model. Ever. Scrolling down to the "Camera" section, again it's very clear that the dual camera on the 7 plus is far beyond the 7, and the 6s plus - offering 2 x optical telephoto at f/2.8 to complement the f/1.8 wide-angle, plus depth-mapped software bokeh, it's simply far more versatile for an EDC camera.
You got what you paid for. The Plus costs USD$120 (18%) more, but gives you 17% more screen diagonally, plus 23% higher pixel density, plus a whole nother 2x telephoto f/2.8 camera, and a whopping 48% larger battery - plus additional features enabled by this (third!) camera. It's clearly the superior device unless you just can't take the size.