They're both 512mb versions, and both have higher cpu and shader clock speeds so they are better than the 256mb version Kemp linked.
The amount of memory really only makes a difference when you run at higher resolutions and/or the game has a lot of texture/etc information which it wants to load into video memory.
Notice the 2nd one you linked, on the specifications page, shows:
Core Clock Speed 660MHz vs 600MHz Standard
Shader Clock Speed 1600MHz vs 1500MHz Standard
this means the card comes pre-overclocked which should provide a boost in performance. Granted you can do the overclocking yourself - if you feel up to learning how etc. But bear in mind overclocking can void warranties. So a pre-overclocked card (costing only £6 more) may be worth the cost in your situation.