The only camera you will ever need???
The Rollei will be easier to handhold with slow shutter speeds as it has no moving mirror. You also don't get viewfinder blackout on the exposure. So it can be a great street/people/party camera. It is also lighter and smaller and quieter.
The Hasselblad will allow you to change lenses for when you want a wide or telephoto. You can also get spare or different back for it.