Here is a great writeup that I highly recommend you take a look it. I've tested a couple of the scopes he talks about here and my experience mimicks his. So it's not just some bs made up because of brand loyalty like most of this thread.
http://opticstalk.com/inexpensive-sc...opic21176.html
That being said, I wanted to review a Nikon Prostaff also and see if my opinion was the same as his, so I did, and sure enough here were my thoughts on it.
http://opticstalk.com/nikon-prostaff...opic25460.html

The Burris FFII is my favorite scope I've used in that price range range, but I've not used the Vortex Diamondback. Another option to consider is right now some of the Vortex Vipers are on clearance, and they are a much much better scope than any of the ones you listed.