Unless you need a 17" or 23" the Specialized is OOS at Evans; also the price is £369 for the Specialized, and £449 for the Diamondback

For which one, would go for the Specialized everytime over the Diamondback, despite the seemingly lower spec.

For the frame, the Specialzed will be the better of the 2 options, from the look on the pictures of the bikes, the Specialized look more like a higher end bike from the geometry / setup that the Diamondback; the top tube on the Diamondback looks to be very high for a modern bike.

For the spec, RS Darts are better than the Suntour's on the Specialized, but not enough to worry about, they (the Darts) are servicable, but the cost may not make it cost effective. For the brakes, yes the Diamondback has better hydraulics, but if you are going to get cable, Avids like the Specialized has are the best to get (BB7's would have been nicer)

Both shops are good, Evans has an advantage if you have a store near you, as you can get it shipped to them, and they can build it up, with Winstanley, they will ship built / checked, but it will be upto you to make any final adjustments.