The 8500DV is not clocked as high as regular retail Radeon 8500 cards, although it can be easily overclocked to those speeds. The 8500 128MB is clocked the same as regular 8500s.
Also, if you do or plan on playing games that are texture intensive, the extra 128MB would be a good asset.
While whether the 128MB version is worth the extra money is questionable, it is definitely the better card.