I'd go with 8500. You can simply run it the same as you would have the 6400, but the option is there to upgrade your processor slightly later without having to replace it. Plus I figure it's better to run better quality kit at lower performance than lower quality kits at full performance.
£5 to save buying a whole kit if you upgrade in 18 months time? Yes please.
£5 to save buying a whole kit if you upgrade in 18 months time? Yes please.