Ive used both and owned both more than once. They are quite different and I would decide firstly depending on what I was going to do with them.
And if you decide BMCC I would also have the BMPCC in mind.
You dont "Need" any accessories for the BMCC, but it can sure be "Nice" to have. But even if the NX1 is a bit more out of the box, I used pretty much the same number of accessories for it as on a Blackmagic.
Im not talking about you know, but many people in got confused when the BMCC/BMPCC was new and said, "No I dont want a Blackmagic because Philip Bloom, DSVLvideoshooter, etc, said it needs a rig to operate, so Im buying a DSLR instead".
Well, those guys uses rigs for DSLRs to, and for video cameras, and cinema cameras.
So imo and experience you can just pop on a lens and ND and go shoot. For the NX1 you bring extra batteries, for the BMCC you bring a battery.
Tripod is needed for both in most scenarios but the NX1 is of course easier for handheld without support.
But if you just want to know the difference in image and output, my personal opinion is:
+ Very small files
+ EVF/Tilt Screen
- Need conversion
- Sometimes to sharp (looks to video)
- Need the expensive S-Zooms imo (which are awesome lenses and can be had cheap, I payed 0 dollars after selling)
+ Awesome and cinematic image
+ Higher Dynamic Range
- No Slowmo
- No EVF
For narratives I would easily choose the BMCC over the NX1.
If you want a camera for handheld thats a bit bigger than the NX1 but smaller than the BMCC. That shoots amazing cinematic Raw as well as compressed HD still worthy of webb publishing. Better DR than the NX1 but a bit lower than the BMCC. Long lasting battery. Can take stills. Shoots Raw slowmotion. Costs about the same used.
Then the 5Dmkiii is for you.
(BTW, regarding the fact that the NX1 is off the market. I wouldn't care to much. Any of the cameras will loose a lot of value over the coming years anyway. I recently put out a WB ad for a NX1 and was offered the body in mint condition including the S-Zoom and a couple of primes for less than $1000 from more than one seller.)