Looking to add an ultra wide prime either to expand the wide range of the 20-70, or to take by itself when needing to travel light. Given the size, price differences, is there a significant IQ difference favoring one over the other? Or perhaps the Sigma 17mm F4?
Lens will be used for landscapes at mid aperture, so wide aperture lens isn’t needed.
Appreciate any recommendations.
I think the 14mm is demonstrably the better performing lens for IQ, sharper with less distortion. It's actually a stand-out in the line up, a really good lens.
But you need to use it either with rear filters which are an absolute pain, or a 100mm square front filter with a holder which is much better, but still less convenient than magnetic circular filters, you also lose the possibility of a CPL effectively.
So I'd say the arguments for your use against are that the 14 is filter-awkward and that is designed for astro as well as landscape and therefore you are paying for an aperture you won't use much (though the 1.8 can be very useful), even though it is the better lens. And a great landscape lens.
The 16mm will be fine for quality I have no doubt (I haven't owned one) even if it comes second to the 14mm when pixel peeping, and allows 67mm filters - the only consideration for you is whether 20mm - 16mm is worth the effort. If you were using the 24-70 it may make more sense, but with the excellent 20-70, maybe there's an argument that the 14mm would give you sufficient difference from your existing kit to make it all worthwhile?