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 just went from Epson 5030ub to the Epson 6040ub. In my humble opinion the 6040ub has a better picture and I'm happy I changed. That being said, I'm a novice when it comes to projectors.
No doubt about it, upgrade the projector. For $5k you will be able to buy a used Sony 4k projector. I recently went from HD (Sony VPL-HW65ES ) to UHD/4k (Sony VPL-VW665ES). The difference in resolution and general image quality is staggering.
@auxinput: thanks for the advice. SI and Stewart are both 1.3 gain however I've seen Stewart studiotek screen and it is way smoother than my SI. That's why I wanted to know if swapping screens would make a big difference. Ive also heard from 2 different resellers that upgrading screen is more important. Have you ever A B'd screens with same projector? 

@ricred1 What screen do you have ?

So screen quality does not matter as much as claimed by a couple of resellers of screens I spoke with !?