Still trying to work on my hold .....

What is the easiest way to find out which hold works best for a shooter? How many range sessions it normally takes? I am still experimenting between center hold and six oclock ( with a thin line).  On one hand I see my sights way better against white paper. On another hand center hold is more relaxing and natural for me.

I would think you need to choose one and shoot it for a while. By that I mean several sessions. Then if you want to compare, convert to the other, also for a number of sessions. The answer should become clear a few sessions into round 2. You will either decide you like the second, or convert back to the first, but one session each back and forth I don't think will yield a clear result.

I need 20-30 sessions to properly evaluate a change. 

In my experience center and 6 seem like they should be better but they produced more flyers. IMO deep sub-6 is best for all beginning and intermediate shooters. If a shooter has a 10 ring hold, excellent trigger and trusts the process enough to ignore the bull in the background, then a 6 o'clock hold might work better. That is why many top shooters use it. For us mere mortals clear sight alignment is more important.

Center hold works well for action shooters who value speed over accuracy. I don't want to give up the visual clarity of the sights, especially indoors.

Does the skill level matters in this conversation ? I am totally new to BE shooting. ( not new to shooting )   Using a full size 1911 with irons and full power duty ammo , I am usually around 250/300 on NMC.( 50 /25 yards ) using center hold on both. And I am kind of stuck there. Anything I can do to improve it I will try. Anything will help.

I would suggest whatever helps you see the sights better and more consistently.  The target does not matter.  Everything that determines the size of the group happens at the gun.  The target is just an Area of Aim, not a Point. 

I'm old enough and have enough correction that the only thing I can really see is the front sight.  The area of aim is just a white place under a fuzzy grey circle. 

CR

"The target is just an Area of Aim, not a Point" written by CR10X is excellent advice.
Phil