I'll start this.

[Show me]

I always install one piece rails.
I scope hop a lot and it makes it much easier. Plus it gives me the illusion that all is perfectly lined up.

 

[CFJunkie]

I tend to go with separate rails, simply because I do a lot of single load shooting and I find that one piece rails get in the way of loading single shots. Most of my rifles have had a lot of use and the chambers have eroded so I am seating out pretty far. Also, with heavier bullets in any caliber, the heavy ones are much longer, especially in a .223 where the light 52 gr bullets are about 0.7 inches overall and the heavy 90 gr bullets can be 1.24 inches long.
With my old fingers, the one piece rails get in the way when trying to single load.