which is a better option for portraits Nikon d810 or canon 5d mark 3?

man

A new camera body won't change a lack of skills .

I've done studio work with a Nikon D5000 and the 18-55mm kit lens and got magazine quality results .

I'd suggest you learn to use what you already have .

You should explain why you not impressed with this setup, what the lack of d7200 compare to ff camera.

But if i were you, i wouldnt upgrade my camera and only use that lens, it is a good lens for all around ff, but that it.

D7200 has quite good dynamic range and tonal range that you can pull it in editing. The key for portrait are to know the full manual setting and editing, and of course selection of lenses that can deliver your taste, because every photographer have different taste in lens character.

Try some lenses that can make a good portrait that suit you.

Start from 50mm 1.8/1.4, 85mm 1.8/1.4, 135mm f2 dc, 70-200 2.8 ( nikkor or third party like tamron ), try it first before you buy it. And decent flashes for strobist when you need artificial light.

Using a "portrait" lens on your d7200 should help a little,

but what will do the Most good is Improving Your Knowledge And Skills.

In photography, the most effective way to get better results is for You to get better At it.

You can shoot anything with any dSLR. It all depends on how you set it up. Portrait shooting is best done with a short to medium length telephoto lens. The focal length you end up using depends on how far you wish to be from the subject.

YOUR skills are the problem, not the camera and not so much that lens (I no longer believe in third party or generic lenses from ONE solitary but very embarrassing experience I had with a brand new Tamron lens, which I sold at a loss to buy the Nikon lens I still own).

Some people swear that the generic brand lens they have is "just as good" as the brand lens that chose not to buy, which causes me to question what they SAY they "know."

There is not much difference between the quality Nikon 810 and Canon 5D cameras. They're both great cameras but if you are going to upgrade thinking you skills will automatically improve... you're sadly mistaken and you'll be very, very disappointed because you'll only be making the same mistakes with more expensive cameras.

There's nothing wrong with your current camera but you may want to replace your generic brand lens with a NIKON ("Nikkor") lens; you'll quickly see the difference.

INSTEAD of buying a more expensive camera, why don't you simply invest the money AND time and take a bona fide, organized Beginner's Photography class? You'll learn more in those 12-14 hours of instruction than you'll learn on your own going through 5-7 months of disappointing and discouraging trial and error. You'll understand how to use light and shadows, how to compose, you'll learn about the uses of different lenses, too. Give it some serious thought, won't you?

I also think you are going down a totally wrong path thinking about another body. There is SO much more that goes into good portraits than just a camera body.

First of all, the main thing a full frame camera is going to give you over a crop sensor is more shallow depth of field for any given focal length. If you go full frame, your 24 - 70 lens is not really a good portrait lens. You need a high quality 85mm and even a 135mm can do well. In fact, for Canon full frame, the 135 f2 L lens is one of the best portrait lenses in existence, but it cost nearly $1000. The 85mm 1.8 is NOT a pro quality lens. If you get a Canon 5D Mk3 and put the 85 1.8 lens on it, you are going to be pretty unhappy with your results. You need a lens, (either Canon or Nikon) that is sharp wide open, or at least sharp stopped down a tad.

Lighting is the other critical aspect of good portraiture. What are you using for lighting? "Natural light"? Are you using fill flash? Is your lighting OFF CAMERA? Do you know how to control focus point? Do you know how to achieve correct white balance / skin tone?

Good pro quality portraits do not automatically happen by simply buying a certain camera body. If you cannot boil a hot dog, going out and buying a massive, expensive set of pots and pans is not going to magically make you a great chef.

Reading your question makes me feel you have a photography education / lack of skill problem far more than a camera problem.

A new camera body won't fix skill issues. Are your skills and lighting kit up to par? Hard for us to know since we don't know your work.

If you really are pining for a new camera body, I would stay with Nikon since you already have money invested in a decent lens. If you go the Canon route, you'll need to get another lens kit, perhaps an 85mm f/1.8. A 50mm lens on a full-frame body would be better for full body or small group shots.

Are you processing raw images or just using the JPEG files the camera produces? If you're using JPEG images from the camera, that image is what the designers think the image should look like. Nikon and Canon have their own ideas what the processed JPEG image should look like. If you are shooting raw, then you have control over the final image processing.

There is also post processing skills need to bring out the best in your images. These are generally small adjustments to some of the exposure controls to bring out the feeling you want. Adobe LightRoom is a wonderful tool for doing this.

Have you considered that a different lens, and better lighting would probably pay off dividends rather than getting a new camera!

You might want to think about what is is with the images you've taken that is wrong, anf then think about fixing that!