I'm torn. I'm ML but I'm all about that leftist unity and I love my anarchist comrades. So in that sense, I do really prefer the red/black flag.
But then again, damn that all-red flag just looks so damn good. It's simplicity really gets the point across. And the all-red flag does have some links to the anarchist movement in the 19th century, IIRC. Not to mention fucking ancaps have co-opted the black for their lame ass yellow/black flag. It sucks but I despise them so much.
I dunno, what do you all think?
There's a bunch of dope songs in different languages about the red flag:
The Red Flag (English) - Original Version, Alternate Version by Billy Bragg
Bandiera Rossa (Italian) - Original Version
Le Drapeau Rouge (French) - Original Version
Probably more that I don't know off the top of my head
So personally that would tilt things in favour of the plain red flag for me.