Since the choice of lens is highly subjective, we can’t tell you definitively which to pick. Instead, I’ll offer a suggestion about how to make the decision yourself.

Each lens has many variables that can be evaluated only with the specific lens: sharpness, bokeh, speed, physical dimension, weight, etc. But one thing you can test quite generically is framing.

A 50mm lens will give you tighter framing than a 35mm lens.

Before moving to (crop-sensor) digital, my go-to lens was a nifty-fifty on a film camera. On the digital camera, it felt very uncomfortable. 35mm felt a lot better. After learning about the difference between crop sensor and full frame, this made a lot more sense.

Dig out your old kit lens or get a cheap 18-55mm kit lens second hand. Then zoom it to 35mm and look through the viewfinder, maybe even take a few snaps. Then repeat with 50mm. This would give you a sense of which you prefer.

After that, treat the zoom as just a zoom and do a just-for-fun photo shoot. Then collect your photos and see what focal length your favourite shots were set at.

This should give you a good idea of which lens to go for.

Note on sharpness and bokeh: lens reviews on the internet cover these extensively. In picking the 2 lens options, your research should have already covered both aspects.

If you still find it hard to decide, I suggest going for the 35mm lens. Your existing 70-200mm shouldn’t be too far off the 50mm prime, so the 35mm prime will give you a more ‘different’ feel to the zoom lens.