I've spent a few weeks shooting around with the Nikon Z6. And in that time, I've made up my mind that as a current D750 owner of 3+ years, I will not be trading it in for its mirrorless sibling. But maybe you should. Here's why...

Marginal size/weight advantage

The Z6 offers a size and weight advantage over the D750 but it's less than you might think. At 675g (with battery and card), it's about 20% lighter than the D750 (840g). And though the D750 feels a little chunkier in hand, I prefer its grip. I can get my pinky on it comfortably and access the front function button with ease. On the Z6, there's no room for a pinky and its two front function buttons are awkward to reach. Of course, everyone's hands are different, and the Z6 might fit more comfortably in your hands.

Little image quality advantage

Raw image quality between the two cameras is nearly identical, save for the lack of mirror/shutter shock at some shutter speeds on the Z6 as long as you enable 'Electronic Front Curtain Shutter' (which you should). As a music photographer, I shoot a lot of personal work in low light environments. And in extreme cases of shadow pushing, the Z6's on-sensor AF points can show up as banding. This isn't the case for D750 files. Though if you shoot JPEG, the Z6 files are much nicer.

Less reliable low light AF

Shot on the Nikon D750. I love to shoot live music in underground/DIY venues, like basements and living rooms; places where lighting is dim and flash photography is generally unwelcome. The D750 is my go-to camera for this and all kinds of still photography work because it's reasonably small and light, offers great dynamic range and, most importantly, has rock solid low light autofocus, not to mention reliable tracking.
ISO 3600 | 1/320 sec | F2.2 | Shot using the Nikon 35mm F2 D

In my experience, the D750 is a more reliable autofocus performer in dark conditions than the Z6. Nikon even rates the D750 as 1EV more sensitive, -3EV vs -2EV, when using AF-S (the Z6's 'low light AF' mode is sensitive to -4EV, but Contrast Detect-only). What's more, this rating only holds true for the Z6 for F2 or faster lenses, while the D750 will happily focus at similar light levels with F4 zooms. Simply put, I found the Z6 hunting in conditions the D750 doesn't. And when using the D750's central cross-type points, it has proven to be a top performer, whereas initial impressions suggest the Z6 is about as good as the Z7: which is to say, not great.

Difficult to use AF tracking

While I stick to a single, central point on the D750 in very low light, I like to use 3D-Tracking in brighter conditions. And it couldn't be easier to use – simply hit the AF-on button and watch the camera track whatever is under the current selected AF point. From a compositional standpoint, the AF focus area can be somewhat limiting, but the reliability of the system is top notch.

With the Z6, the 273-point system with 90% coverage offers virtually no compositional barriers but engaging tracking is needlessly cumbersome to do when shooting with an eye to the finder. It's only available in the camera's Auto AF area mode and requires you to first press the camera's 'OK' button in the center of the directional pad, which I can't quite reach without compromising my grip. Worse, you have to press 'OK' again just to switch subjects, something that is simply too difficult to do when shooting fast-paced subjects. This is a shame because Nikon DSLRs sport the best subject tracking mode in the industry: 3D tracking.

Shot on the Nikon Z6. As a Nikon DSLR shooter, the Z6 has a familiar feel to it. Still, I prefer the reliability and usability of the D750's AF system to that of the Z6 (even if the point coverage on the Z6 is way better), as well as the D750's greater usable dynamic range.
ISO 100 | 1/320 sec | F4 | Shot on Z 50mm F1.8 S

Limited support for Nikkor AF-D lenses

Almost all of my lenses are Nikkor AF-D, which use a Nikon DSLR's internal focus motor to drive focus rather than having a motor in the lens. These lenses represent excellent value for Nikon DSLR users and are generally very fast to focus, though noisy. Jumping into the Z-mount would render these lenses useless from an AF standpoint, as Z mount cameras have no motor. However most newer Nikon lenses have built-in motors and work with no noticeable penalty adapted via the F to Z adapter.

But...fabulous video

Video is the one area the Z6 has me hung up because it absolutely smokes the D750. If you have even the slightest interest in video capture, the Z6 is a far better choice than the clunky, prehistoric video offering found in the D750. The Z6's 4K quality is excellent (sorry, only 1080p on the D750), in-body image stabilization is super useful (not available on the D750) and the camera's touchscreen for AF point placement (also not available). Focus during video is solid on the Z6 in contrast to the downright awful Contrast Detect video AF found on the D750. And did I mention it shoots 10-bit log over HDMI?

Bottom line

As I'm primarily a stills shooter, at the end of the day, image quality and AF reliability (especially in low light) are the two things that matter most to me and the D750 holds its ground in both regards to the Z6; all the touchscreens and in-body IS systems in the world can't change that. For these reasons I'll be holding on the my D750 and waiting at least another generation before making the move to Nikon mirrorless.

But those interested in video would be wise to give the Z6 some serious consideration because it really is that good. Plus other features, like better AF precision without a need to micro-adjust, live exposure and depth-of-field previews in the finder, and greater AF coverage, might also win you over.

If you're even remotely tempted, consider this: depending on the condition, you can sell your D750 for upwards of $1000, doing so would put you halfway to a new Z6. Just don't forget the adapter...