Upgrade the projector or projector screen?

Hello Everyone!
 My Current setup: 
Epson 5030ub + Screen Innovations series 3 - solar white (1.3 gain) 100 inch diagonal.

I sit about 8 feet away from the screen and my HT setup is totally dark with grey walls. 

My goal is to upgrade the picture quality and I want to stay under or around $5k. 

My QUESTION: Should I upgrade to a Stewart Filmscreen StudioTek130 screen OR upgrade my Projector to a JVC DLA x790r or similar? 

What will make the biggest positive impact? 
I think your screen is very good and your projector is too.  If you really want to upgrade now, then the projector will make the biggest impact.  if you had room lighting or a very crappy screen now, i can see how a better screen would make sense.  personally Id wait a year to make a bigger jump but either the sony or the jvc 4k projectors will be a decent upgrade if your sources are hdr.

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Why would you buy a white screen for a dark room? Your blacks are only as black as the screen, I would have gone with a 0.7gain or similar if my projector could handle that. It’d be like going from LED to OLED.

That said, not sure you should upgrade that over the projector, though the Epson is good.
Maybe he wants white screen vs gray..  That way the  whites are white and the colors are vivid.  Grey is gonna dull it down a bit on colors. Grey is good if room has ambient light, but you get duller colors and off whites.
my jvc dla 4k upshift projector doesnt have any problems doing black onea 1.1 gain white screen by draper in a dark room. 
In today reality, when the techs are getting old too quickly, I would suggest you sell the current one and buy something newer, for example, BenQ MW535A, it's well represented in this overview, but other options are also viable, at your preferences!