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.

 

[Txhillbilly]

I use one pc rails on almost all of my rifles. Mainly because I shoot long range and it's easier to find 20 - 30 moa rails than it is to find rings that way.

 

[K9TXS]

I prefer the one piece rails as well. No particular reason, I can think of other than that's what I always have been using. Somehow in my parts box I have some 2 piece rails still in the packages. No idea where those came from.

 

[SevenFan]

I prefer one piece as well, the only one with a 2-piece is my 1957 110.

 

[CFJunkie]

With the 2 piece rails, I use Burris Signature ZEE rings that have plastic inserts that can be changed from -20 to plus 20 MOA in 10 MOA increments.
I admit that if I were shooting at 600 to1000 yards, I would probably be using a single piece rail.
But with my .223 set up for heavy bullets, I would have a hard job to single load the 90 gr bullets and get them into the chamber.
Even with 2 piece rails, the extremely long Berger 90 gr VLDs are so long when I seat them out to get close to minimum jump, I can single load them but once the bolt is closed, I can't eject them without the firing because there is still bullet tip in the chamber when the bolt is completely back. I have to remove the bolt to get an unfired round out.