Fixed/set scrolls pins you to the map's dominant (not average) BPM, and treat them like it's scrolling with x speed at 100 BPM (after I tested, maybe, maybe not?). The formula is:
fixed/currentBPM = target/100
target = fixed * 100/currentBPM
Please correct me if I'm wrong.
BPM scales just directly multiplies current BPM (regardless of dominant BPM) with your scroll. Be it like, X speed on DDR/ITG/PIU series.
Fixed scroll:
+ Constant reading (in dominant BPM of the map)
+ Easy to use
- Inconvinent on gradually-rising BPM
BPM scale
+ Dynamic reading
+ Easy to tackle maps with heavy BPM change
- Need longer time to set up
- Every map has different scroll speed with this
The choice? Your call.