In this case which would you think be more significant, getting a d750 for my nikon 85mm 1.8g lens or getting a sigma art 50mm 1.4 lens for my current D5200?
I would say get an 85mm lens for the D5200 instead. You get a 135mm focal length equivalent, while on full frame, it's ideal as a short portrait lens. I imagine you already have a 50mm f1.8 lens, so that's more than fine as a short portrait for the D6200, and will serve you well if you opt to move to full frame at a later point.
I have an 85mm 1.8g for my d5200 but it's too far for my liking and hard to nail focus, I also have a 35mm 1.8g DX to make it a combo, not sure now the next step getting a sigma art 50mm 1.4 or a D750 would be better. I also have 2 DX zooms 2.8 (17-55, 11-16)
An 85mm lens is hard to nail focus wide open. If after stopping down to f2.8 and it's still not focusing correctly, then you should send it in for calibration with your lens body. But if at wide open, you'll have to lower your expectations for getting critical focus every time.
As to your original question, I think it boils down to how much portraiture you do, and what kind. I agree that the Sigma 50mm f1.4 Art is a fantastic lens. But if you already have a nifty fifty, ithe Sigma is not going be that much better for portraiture. Sharper, yes. But portraiture isn't about sharpness, which is why I now use a Nikkor 50mm f1.2 AIS instead when I can find time to manual focus. It's quite soft and hazy wide open, but that gives skin tones a very flattering smooth glow. Using a very sharp lens for portraiture is going to show a little too much detail, right down to warts and skin pores, which if isn't softened in post-processing, is going to be very harsh on your subject.
I switched to FX out of long-term economics and necessity, not because I wanted a full frame badly. In fact,I was dead set not to go full frame for event shoots, because I no longer like lugging a DSLR around when I'm not shooting for work. I hated having to sell off my Canon 70D, as I already had all the glass I needed, and if it wasn't a bad copy, the 70D had everything I wanted in a workhorse.
I opted for a D750 over a D7100 or D610 because it represented better savings in the long run. The D610's kit lens is a 24-85 f3.5-4.5, which means I'd still need to get a telephoto lens immediately for work. On the other hand the D750's 24-120mm f4 kit is all I need for my event shoots, so a telephoto lens isn't mandatory yet.
I didn't go for DX because I don't like the Nikon 17-55 f2.8, which is basically a Sigma 17-50 f2.8 at twice the price for no VR, I was previously using the Sigma on my Canon and loved it. But since the reason I couldn't get my faulty 70D exchanged because Canon blamed my AF issues on using a third party lens, I would rather not migrate to Nikon and bring exactly the same problems over. That means using only Nikon glass for my work shoots. I would get third party brands if I don't need it critically, like say a Sigma 35mm f1.4 Art but as much as I like the Tamron 24-70 f2.8 VC for example, I can't risk it becoming an excuse for Nikon to deny me customer service.
FF is better.
