So I haven't seen the model 3's Obsidian Black but on the S and X (and any cars when comparing flat vs metallic), flat black will look better (in my opinion) right off the show floor and if you baby it to no end. It's black like a baby grand piano. It will show chips, streaks, swirling, dust, dirt, and pretty much everything that gets on it. That's OK for some people, but it can be a whole lot of upkeep to look perfect.
Obsidian black on the other hand is more resilient to that sort of thing. You still need proper care for it to look nice, but the swirls and streakyness are less noticeable and you won't need to wash it every other day to keep it top notch.
Some people like the obsidian over the flat for looks, though. It reflects light a lot more brilliantly I'd say? It gives a bit more depth and feel to the car perhaps. It's still pretty subjective, but if you want less maintenance and worry about keeping it pristine, the Obsidian is probably a better choice. If you are going to baby it either way, then either could be good choice.