Monty from Xiph is on record saying that even no dithering is perfectly fine for 16-bit audio. This is probably true. The quantization noise is really quiet, you simply aren’t going to turn your equipment up enough to hear noise at some low level like -96 dB, or wherever the quantization exactly ends up being. There is probably enough background noise in the signal to begin with, to make the whole question utterly irrelevant – if there is already a broadband hiss at, say, -80 dB, it hardly matters if you add tiny amount of additional quantization noise at some -96 dB level. It’s just going to vanish there and you can never tell it was present in the first place.

That being said, I do recommend use of dither noise, and triangular dither is not bad. These settings you have there control the noise shaping, which can somewhat extend the retention of signal at hearing-criticial 1-2 kHz region at expense of more noise in less-critical bands. I think any choice, including “None” for flat dither noise, is perfectly fine. U shape dither is likely technically the best choice, suggesting by name that it places dither noise at low and high frequencies roughly following ear’s sensitivity curves. If you had some signal near, -96 dB to preserve, then you would likely want to make this choice to ensure that as much of the audio in the hearing-critical bands would be preserved as possible.