I think better deals can be found outside of those two offers. If you could raise your budget a few hundred pounds you would be getting a hell of a lot more bike for the money.

Oh yea, I also wanted to say that if you are planning on riding a lot in rough mountain terrain that neither of those bikes will hold up without significant investment in replacement parts. I bought a bike specced similar to that diamondback a little less than a year ago. After about 3k km of mountain riding all that is left of the original parts are the brakes, frame, handlebars, headset, front wheel, and seatpost. A few things replaced were luxury, and hadn't broken, but many things were necessary to stay safe during the kind of riding I like to do. Had I had the money then to buy a 1k euro bike instead of a 400 euro bike it would have been money well spent