IMO...
Speaking only of OWB quick-draw rigs... it's the firearms position relative to the body upon draw vs. firearm angle relative to the target.
With a forward cant, the gun is already angled muzzle forward... less than 90 degrees, but with a rearward and upward pull towards and perhaps even behind the hip.
With a rearward cant, the pull places the gun forward of the hip, but the muzzle must be rotated over 90 degrees to target after you clear leather.
Which is better? I believe that it really depends on the individual. It takes a little more hand and wrist control to rotate the gun from a rearward cant, and more elbow and shoulder control to bring the gun to target from a forward cant.
I've always been a little faster with a modest forward canted holster, placed in the 2-2:30 position (I'm left handed, but I'll use right handed O-clock position to avoid confusion) because of limited hand/wrist mobility.
And then there are the Berns-Martin design "break-front" holsters.
Do some dedicated practice for at least a few sessions from both positions, actually all three if you've not worked with a no cant rig, and you'll figure out for yourself which is quicker for you.