I've tried both (Hik and Dahua).
Both work well.
If you're planning on using 12MP fisheye cameras (as I do) then you have to go Dahua.
I picked up 128 channel NVR and am slowly moving my analog stuff to IP. So far I'm at 24 cams moved over.
Unlike analog, you need to consider total bandwidth.
If you're planning on using high MP cameras with decent framerates, you might be looking at 4-10Mbps per camera. If you get a budged NVR you might find yourself not being able to max out the NVR because of bandwidth or have to use 2MP out of 4+MP cameras just to get under the bandwidth limitation. Don't ask me how I know :-(

On my 128ch NVR my max bandwidth is 384Mbps, which means that @ 5Mbps /camera I can do max of 72 cameras, 36 if I get cameras that do 10Mbps.

Also, cameras and Onvif is getting better.
I have a mix of 4-5 different OEM MFGs and have all of them working well on the Dahua NVR.
I would expect same on HIK NVR.
There are some analytics, motion detection, line crossings, etc. that may not work natively, but there are workarounds.

I think that would be Mbps (and not MBps.) it’s confusing that throughput is typically stated in terms of bits, but storage is stated in terms of bytes.

You're right.
I've corrected my post.

Hmm, I’d like to understand this better. I have a Hikvision DS-9616 NVR in a project and I was hoping to add a fisheye camera - ideally 12MP. What limitations will I run into?

Regarding the OP, I think the DS-9616 if a great bet if you want RAID, dual NIC and other features for robustness.