Factory overclocked editions aren't worth the extra money, you can overclock them yourself in 3 seconds using the ATI Catalyst Control Center included when you download the drivers for your video card, and you can overclock at a much higher clock rate to get a much better performance. Although, with a HD 5870 you wouldn't need to do overclocking as its already a very powerful card
Go with the Diamond, you wont regret it, and the benefit of buying at a local store (assuming that Fry's is a local store) is that you can return it within 30 days if it fails or is bad instead of RMAing it through the mail which will take forever, and get a new one the same day. Sometimes the local store might even RMA it for you if its past the warranty...