What's the difference..

[Cynphur]

in this card:

http://www.overclockers.co.uk/showproduct.php?prodid=GX-153-OK&groupid=701&catid=1914&subcat=1812

and

http://www.overclockers.co.uk/showproduct.php?prodid=GX-082-GI&groupid=701&catid=1914&subcat=1812

I'm curious they both seem to be almost the same price except one sports '1536MB GDDR5' whilst the other is '3072MB GDDR5'.

Is it an obvious decision to get the 3GB card or is the 1.5 right up there with it?