Which of the two is a more useful to assist in pulling from the floor with deadlifts - stiff-legged or snatch grip deadlifts?
Depends on your weaknesses.
SLDLs - ham heavy.
Snatch Grip DLs - back heavy.
Like @T3hPwnisher said, it depends on what’s weak.
Edit: I’ll add that if your weakness is just pulling from the floor in general, deficit DLs would beat both of these, IMO.
Improving bracing and technique will help more than variations. Address this before worrying about weaknesses.
Why not a snatch grip SLDL? I do snatch grip RDLs (same thing basically) and they help my squat and deadlift, one of my favorite assistance lifts.
Are these just done the same as a normal RDL, but with a snatch grip?