Nintendo

If you have to prioritize one feature over the other, my personal suggestion is to prioritize 4K and HDR over 120 Hz. You're going to get 60 FPS for the vast, vast majority of Switch 2 games. Currently only one title in the launch lineup supports 120 Hz: a single game in the Welcome Tour. Mario Kart World, their premier title, is 1440p at locked 60 FPS in docked mode.

I suspect Switch 2 games in 120 Hz will be only slightly more common than PS5 games in 8K: Sony eventually removed the 8K logo from the box because basically no developer could achieve it and virtually no TVs supported it.

There may be game-specific performance modes with lots of caveats, like 120 Hz at 720p in docked mode for single player, but you would think you would see that with the showcase title. Instead the frame rate for MKW is crushed when three or more players play via couch co-op.

I don't think it's possible to test this at the moment as there's no actual game out right now that uses both HDR and 120Hz at the same time.

It’s unclear to me if HDMI 2.1 is supported at all because there’s no VRR while docked according to specs. I connected mine to 2.0 port and it says it’s capable of 120 Hz at 1080/1440p. I doubt 120 Hz with HDR would require HDMI 2.1 bandwidth since 2.0b can do 4K60 with HDR.

I'm running a QD OLED monitor, so take that into consideration with my results. HDR in Nintendo titles seems to be more focused on increasing colour accuracy rather than "pop", which will be something that is down to your personal taste. For example, the sun in Breath of The Wild turns from a white blob in SDR to a well defined sun with rays in HDR. It is definitely noticeable, but not in the same way as a lot of other consoles or PCs do HDR. However, this depends on the game, as Cyberpunk 2077 (I don't own it so have to go off what other people say) has a far brighter and more traditional HDR master that has high contrast.

I would not recommend getting a HDR 400 monitor if you can. The specifications for HDR 400 are significantly watered down (10 bit not required, sRGB colour gamut rather than WCG, high black level luminance allowance, so on) compared to even DisplayHDR 500 certified displays. If HDR is a priority, it would be worth getting maybe a lower refresh rate monitor but with at least DisplayHDR 500, as the HDR will be far better.

I can't comment on HDR at high refresh rates as I have no games at the moment that use both, although I assume it would work fine at high refresh rates.

All I can tell you is that I've played HDR PC games and the difference is noticeable. If the Switch 2 can do it, it'll be worth it. But maybe wait until there's an actual HDR title that comes out?