Nikon 35mm 1.8g vs 50mm 1.8g?

I already have a 18-140mm f/3.5.
I wanna buy a prime lens, which one do you prefer.
I have a crop sensor camera (D5600).

The cropped sensor means that the 35mm lens will closely approximate the view of a "classic" 50mm in the days of 35mm film SLRs.

The reason the 50mm was a popular choice on the old 35mm film SLRs was that it was the standard default kit lens, and also it had an angle of view which was approximately the same as the human eye so gave pleasingly familiar perspectives to images. That also
meant it was a very good lens for a wide range of situations in the days before zoom lenses were cheap and good quality. On the APS-C sized sensor on your Nikon, the 35mm lens gives very close to the same result. On your camera a 50mm prime lens will as a slight telephoto lens.

On that basis, if it was my choice I'd go for the 35mm lens.

The 35 mm would be a good choice for general purpose. It has an angle of view similar to the eye, so it introduces little distortion. The 50 mm would be a good choice for portraits. It is slightly telescopic and flattens the face to be a more complimentary angle, much like a painting. Also the longer focal length enables to get more of a selective focus to help soften distracting background detail.

You can answer this question for yourself. Set your zoom lens to 35mm and look through the viewfinder. The view you get is the same as that of a 35mm prime lens. Take some pictures of the sort you regularly take (landscapes, street scenes, portraits or whatever), and see if you like the results. Then repeat the exercise, having set the zoom lens to 50mm, and compare the results.

The main virtue of a prime lens, apart from a marginally increased sharpness, is the greatly increased maximum aperture. This has two advantages. You can work with smaller depths of field, which is good for portraiture and the preferred prime would then be the 50mm. The other advantage is working in lower light levels. If you do not want to work in lower light levels and have no great yearning for portraiture then I suggest you hold off buying a prime lens anyway. Zoom lenses are very versatile.

The only reason to go with a prime, over what you already own, is wider aperture. Personally, I would go with a 35mm on a crop sensor camera, but that's my choice. If you're after portraiture, then the 50mm. Of course, I would skip the 50mm and get the 60mm f/2.8 macro. The Nikkor 35mm f/1.8 DX lens has lots of distortion, odd for a prime. It is sharp, but so is the 18-140. As Peter stated, try tape your zoom ring to 35mm and 50mm and shoot for a day or two on each setting.

It's up to you. Do you want a versitle lens or a lens just for portraits? Just get the 35mm 1.8G, for any kind of use.

35mm for "general use", 50mm for portraits. Set your existing zoom lens to 35mm and to 50mm to see the angle of view of those lenses.

On that body the 35mm is closer to a 50mm on a full frame camera.
The 50mm is closer to the 80mm on a full frame.
So it depends on what you want to use it for.
I bought both.