If there is a difference, then there should be a new category. But if the extra memory doesn't give any better performance at the same clocks there's no need for it, then the other category is the right place to post.
...but one point that hasn't been considered often is that more memory COULD mean that those cards will max out at a (much) lower memory frequency than the cards with half the memory. If this is the case, IMO a new category should be made as well.