Idk tbh.
I initially liked the look of Space Grey because of the then new sexy design. Then it got old already after 2 years. I also didn't like at all how the white faced iPhones really received all the love the past few generations, neglecting the black face which was then stuck with SG 2013-2016 - white iPhones got Champagne with 5s, Gold with 6 (just a champagne clone but still), Rose Gold with 6s, in addition to the standard silver, yeah enough.
Anyways,
Black and Jet Black strike a balance between retaining the now iconic (and even timeless?) 6 design while also ensuring both have a very futuristic look to them, featuring how they are a technically modernised version of the 6 - deciding which black to get was incredibly difficult for me but I'm very happy with Jet Black.