What is the advantage/disadvantage of fibers that are 0.029" vs. 0.019"? Is illumination the major consideration? Durability? Cost?

Also, is it normal/typical(?) to see a double pin when aiming on the X?

I like a 0.019 pin size. I tried a 0.010 last year and it was just a little hard to see. 0.029 should be a little brighter but it may obscure more of the target than you want. It just depends on your preference in how you aim.

I prefer .019 pins for 2 reasons. 1) they block less of the target dot, which became an issue when I decided to expand my kill range and began practicing with smaller dots at 40 and 50 yards. 2) my Cobra Sidewinder Radiant sight is so bright, the smaller dot becomes less distracting in low light. Some sight manufacturers seem to think the smaller pins should add to the cost. I found replacement pins for my Cobra for about $10 each, the same price as the .029 pins.

I like 0.019 more, in my opinion it is the best not to big like the 0.029 pins and not tiny like the 0.010 pins i notice that with the 0.029 pins that the pin at certain distances actually covers up big spaces of the target.