All my sights are 5 pin, .019 on the top 3, .010 on the bottom 2. The further you shoot, the more of the pin covers the targets. I use .010 on 50+ for accuracy. Smaller pin, more precise aiming. Some people cant see the .010 well enough and use .019 on everything, or .029 etc. You kind of need to try them out to know what works best for your eyes.
Not sure what you mean by front and back pins. If youre talking about vertical vs horizontal, its mostly sight picture. Vertical pins are going to take up less room in the sight housing as your pin(s) are stacked on essentially 1 sight post. Horizontal pins are each on their own sight post and typically has 3/4/5/7 sight pins coming out of the side of the sight housing. Some people dont like a bunch of clutter in the sight housing. Again, all personal preference.