I don't know how well the electronics work with a Leitax adapted lens on Nikon, but the mechanics are solid.
The rest of this assumes that you're going to keep your Nikon zooms for casual shooting, that you're willing to work with a manual lens the rest of the time, and that you're looking for a moderately wide angle between 24mm and 50mm. As such, you'll probably be using a tripod and stopping down the lens. If that's the case, I'm not sure I can recommend Leica. All of the older wide lenses, including all of the 35's, have soft corners by modern standards, and the modern wides--the 19, 28, and 21-35--are all expensive. The R system is where I put my money because of their holistic performance. If I knew I wouldn't be using a lens wide open, though, I think I'd go for a different route.
The MTF graph at photodo.com for the Zeiss C/Y 35-70/3.4 at f/8 is very, very good. At 35mm and f/8 it outperforms the Leica R 28/2.8, which is among the best 28mm lenses, as well as the 21-35, up to the extreme corners where the 35-70/3.4 is a bit softer but remains competitive. Resolution is flat across the frame until it drops off steeply, though, which may not be preferable to the more consistent performance of the Leicas. The Zeiss doesn't show the focal plane waviness or astigmatism of the Leicas, though, and has comparable vignetting and distortion. It costs about a third of either of the Leicas and also performs equally well from 35mm to 70mm. It just doesn't deliver the same exceptional results at wider apertures and it uses a one-touch zoom/focus mechanism, so it isn't as popular and costs less.
If the Zeiss 35-70 is in the running, the Leica 35-70/4 should also be. It has better corners though is slightly worse through most of the frame and it has more astigmatism. It costs more, but that's for wide-open performance, where it takes the Zeiss for a walk, and two-touch operation.
Note that these zooms came about when zoom lens design was starting to be well understood, and these zooms take advantage of the extra lens elements used in zooming to correct for near-edge performance, which is how they can be better there than most every prime in existence. Nikon and Canon have focused on fast or wide-range zooms (which have to deal with far more aberration than slower or narrower-range lenses), and consumer zooms (where price is more important than optical quality). If Nikon came out with an expensive, slow aperture, and narrow range zoom, that's probably exactly what you'd want to buy. But they haven't, which is why I suggest these Leica and Zeiss zooms for consideration.
That said, the Zeiss 35mm f/2 comes in ZF mount, so the metering and exif data should work like with a Nikon lens, appears to be at least as good as the 35-70/3.4, and slots between that lens and the Leica 28/2.8 in price. The Sigma 35/1.4 is reportedly excellent, and autofocuses to boot, but I don't have any reliable MTF graphs to analyze for it and the large maximum aperture makes me hesitant about it. I don't know anything about the T/S lens, though its larger image circle bodes well for corner performance. As these lenses can be bought new, they can also probably be rented for thorough testing against your workflow.
While I've used a few of these lenses, most of my thoughts here come from looking at MTF qraphs. They're most useful for predicting how in-focus images will look but they still don't tell the whole story, so, of course, look at sample images before buying.
Anyhow--that's my ranting for today about good wide lenses for stopped down work. Let us know what you decide on and how it works out after a few prints!
Cheers,
Jon
