My experience over the years with Sigma lenses (both MF and AF) are that they are optically good but the build quality is often poor and they tend to break rather easily from normal use. First the surface markings wear off and then the moving parts start to break. Maybe the newer lenses are built better but I would never buy one to find out.
I would go for a gently used Nikkor over a new Sigma any day. Its value will also hold much better than a Sigma if you later decide to sell it.
Older, manual focus Sigma lenses had pretty dreadful build quality. This was 1970's-1980's era stuff, and the build quality and materials were sub-par compared to Canon or Nikon lenses. Also, optically, the older Sigmas were a cut below Nikons in many cases-BUT,at dramatically lower price points. Still, a lens is a lens, and even a rather modest-quality lens can make a lot of good pictures. Especially when the "picture" is seen small, or on-screen, or is retouched/modified/edited/enhanced; let's face it, in the 1970's and 1980's we often shot and showed 4x6 inch prints on a regular basis, and that is not demanding shooting. Same today--looking at an image seen a foot across, on-screen, most lenses at f/8 are perfectly fine. I shoot a lot at f/7.1 with flash indoors, at f/5.6 outdoors a lot, also 6.3 and 7.1 and 8...at such apertures, almost ANY lens is adequately good. Diffraction takes the real acutance edge off by f/6.3, and at f/7.1 and f/8 a cheap, $40 used Nikkor zoom looks adequate on a 24 to 36 MP sensor.
As far as AF Sigmas, I have owned a few, less than a handful. 18-125 for APS-C, horrible flare, zoom creep,not sharp,loud focusing on Canon. 100-300 f/4 EX-HSM, 180mm f/3.5 EX-HSM APO-Macro, 80-400 OS for Canon. These were there "good" lenses in the late 1990's/early 2000's era. On Nikon, the 180 macro and 100-300, both their top-shelf lenses, FAILED to focus correctly on newer Nikons. The issue came when Nikon added a second focusing control button on the D200 generation, effectively it seemed, "breaking" the Sigma lens compatibility that worked fine on earlier cameras. By the 2005 Nikon era, the D2x, the 100-300 f/4 EX-HSM and the 180 HSM EX Macro, both part of their "
EXcellence" series models, did not focus right with the Nikon AF-S protocol.
The issue, as I see it, is the focusing protocol...HSM is NOT AF-S, nor is it Nikon's newest focusing protocol,
AF-P. In the future, if Nikon for example, goes to all- AF-P instead of AF-S...I suspect that earlier, older Sigma lenses will not function, or function erratically, with those "future" Nikon bodies. It happened to me. This is the reverse-engineering issue that independent lens manufacturers have to work around.
As far as current Sigma lenses, and the ART series. My opinion is Biting sharpness, hard and sterile bokeh, jarring backgrounds on many subjects like foliage. But high,high test-chart scores. ART means typically over-corrected lens designs that score high lines per millimeter results, but that typically look clinical or hard, and brutal, to me and other fans of soft, creamy, or natural, bokeh. Tamron has gone the same route though with their new 35,45,85 VC lenses...Ugggh...awful, hard,jarring bokeh, especially on OOF foreground objects. Some of the ugliest fashion images I've seen in years were made with the new Tamron 85mm f/1.8 and 45mm f/1.8 VC lenses at a beach in Australia. Tamron's 45 VC looks like rubbish compared to the 45-P Nikkor's bokeh and rendering, but one is an ancient Tessar design and is great at slower apertures, the other is modern, f/1.8, and Vibration Control equipped.
At one time, for 20-plus-years, Sigma lenses had a yellow color rendering. This can _not_ be completely eliminated in post processing software. Lens color rendering is one area where Tamron is closer to Nikon than is Sigma. The issue arrises mostly when you use one lens that's different out of multiple lenses, or mix lenses in a shoot.If you shoot all one-brand, there's never an issue with color rendering.
These days, Sigma prices are higher than they have ever been, but the re-sale is poor compared to manufacturer brand lenses. Especially over 10- to 15-year buy-use-sell cycles; this is where the Nikkor and Canon lenses hold their value better than third-party lenses do.
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