I have a Canon 80D wanna upgrade to Full frame but which one?

I have a Canon 80D
Its been a great camera I'm looking to jump into full frame.

I find myself doing more Weddings and I'm looking getting into product photography in the future.

I have small budget which brings me to Canon 6d or should jump ship to Nikon D750.

Open to suggestions

My vote goes to the Canon.

Take it step by step.
1. Ask "Why change?" Has any customer complained about the results? If nobody is complaining then no need to hurry.
2. Ask "When?" If the camera is still working then no need to rush. Perhaps accumulate some cash instead.
3. Ask "Which" if you are on a small budget then changing manufacturer is a really bad idea, because you have to change all the lenses as well.,
3a. Ask "New or slightly used?" You could save a lot of money by not insisting on new.

For the record, I have bought almost my entire system (Canon) on eBay and not regretted any purchase.

On switching brands:
Do you have good EF lenses? If so, changing brands is a costly move.

Do you have other Canon specific accessories like speedlights and TTL flash triggers? Again, changing brands is a costly move.

You're already used to the Canon system. Get the 6D.

If you re shooting weddings then you need a backup camera. Go for the 6D and keep your 80D as backup.
That way you can share EF lenses (though EF-S lenses are 80D only) and other accessories. They even use the same battery.

This is a fairly large purchase. I would strongly suggest that you go to sites like lensrentals.com and rent both cameras for a weekend or a week. It's not just about image quality. It's about how the camera works and how it does NOT get in your way of you expressing your vision.

Both the 6D and D750 are very good cameras for still photography. Because you're doing weddings, it's likely that you'll be doing video with the camera. With that in mind, the answer is without question, the Canon 6D. Canon is far better for video than Nikon, with the possible exception of the Nikon D500. If you do go with a Canon, you need to download the Magic Lantern firmware upgrade which adds a ton of video-specific features. Nothing like it exists for Nikon.

For these reasons, I'd would go with Canon over Nikon.

One thing you should also keep in mind is: which lenses are you going to be using. Up until recently, Canon's 70-200 f/2.8L was the best of its kind on the market. Nikon's older versions of this lens had very bad focus breathing issues. That's been fixed, but the new Nikon 70-200 f/2.8 is nearly $3,000! The 70-200 f/2.8 is such an important lens, that I'd go with Canon just so that I could use it. Remember, you can always adapt Nikon lenses onto Canon bodies, but you can't put Canon glass on Nikon bodies.

If you want even better video performance, consider the Sony A7R ii/iii instead of a DSLR. All Canon or Nikon lenses can be adapted to work with the Sony mirrorless cameras. It's just that Canon lenses work much better than those from Nikon. Because the A7R has in-body image stabilization (IBIS), this will allow you to get shots simply not possible with either Canon or Nikon. Why? Because not all lenses by C/N are stabilized. With IBIS every lens you use is stabilized. Not only stabilized, but stabilized on all 5 axis as opposed to just 3 with Canon or Nikon.

If video is important to you, then the A7R ii/iii will be the better choice as its video performance is superior to anything from Canon.
If video is important to you, definitely do not go with Nikon!

Will that change, whatever model or brand you buy, improve your photos?