Black is a nice, matte anodized aluminum, much like Apple's iPod Nano used to have way back. It looks sharp, and a lot darker than space gray ever did. Get matte black.
Jet black is what is often called "piano black." It's superpolished, and almost feels like glass, or ceramic, or has maybe a bit like Apple's old-school black polycarbonate iPhone 3G. It also looks a little like the space-black steel Apple Watch. It looks beautiful in photos, and it's a more limited edition that's harder to buy right now.
But it's also a huge scratch magnet. Apple warns about this on its product page: "its high shine may show fine micro-abrasions with use." Scratches happen -- in fact, the old polycarbonate iPhones used to get scuffed a ton. But just a warning: so does the jet-black model. Mine has a few hairline scratches and perma-scuffs ("microabrasions") with a week of mostly careful use.
Does that matter? No. And guess what? Either way, you're probably putting a case on it.
But the jet black is a collector's edition thing, like a special pair of sneakers. It's also harder to find. I'm not into that. Matte black looks great, doesn't seem to scratch, and it's more widely available.
Matte black. That's my advice.
reading•Should you get the jet-black or matte-black iPhone 7?
Mar 19•Having issues with your phone's compass? Read this
Mar 19•You're gonna want the iPhone X in blush gold
Mar 17•Apple event set for March 27, Microsoft creates new division
Mar 16•Apple event on March 27 to focus on education