I have a Metz 45 CT-3 which I like quite a lot, though I've not used it with the Hasselblad yet. It can throw an amazing amount of light. There is a Hassleblad to Metz adaptor that apparently fits onto the left hand grip bracket, and places the flash over the camera. Additionally, Metz has a platform bracket for MF cameras. Either of these is pretty pricey new though. The Metz bracket doesn't seem to be common used. I've sort of resigned myself to using the standard 35mm style bracket with it. There is also a Stroboframe bracket for them.
Some models have a second tube and refector so that you can bounce with the main tube and still have a fill light. I would find that a very useful feature, which is something the Vivitar can't do.
TTL mode with my Nikon works quite well, and gives me perfect exposures from 20-30 feet, a nice option if you have the metered prism that can link to it. The bracket has a tripod screw, which is nice if you are going to put it on a stand.
Downsides, it's a lot bigger than the Vivitar and requires 8 batteries. It doesn't have the built-in tele ability. However if I were choosing between current versions of the two, I'd go for the Metz because of the second reflector. Spec-wise it has a little more power, which is always an advantage.