Well, as the owner of a maxed-out 2015 (M370x dGPU, 16gb RAM, 2.8 i7), who just bought a 2018 MBP Vega 20, I'd say...
Get the 2015. I'm a professional editor, I use the heck out of the 2015, editing RAW 4K video in Davinci Resolve, Premiere, as well as Photoshop and After Effects. My 2015 actually keeps up with all that, but I figured it's almost 4 years old, battery is worn, maybe just get a new one. But...
The new one, aside from looking cool, basically gives me very little I didn't already have, and actually has WORSE storage, because I keep a 256gb JetDrive in the SD port of the 2015, and the new one doesn't HAVE an SD port.
The kicker though, was the fans. The new one runs much hotter, and the fans spin up louder than on my 2015, especially when editing video. (Keep in mind, though, the 2015 can't handle h.264 4K, so if you're editing GH5 footage, it'll lag. I'm not sure if the Vega would keep up with that footage either, though. Better to transcode it to ProRes.)
Anyway, tonight, I'm erasing the Vega 20, and bringing it back to the Apple Store. Nice computer, but it gives me little I don't already have, and I have other ways to spend 5 thousand bucks!