The Nimbus MUni frame allows more clearance for a wider/taller tire than the Nimbus cruiser frame. The rim is much more substantial, and the cranks are much stronger. If you intend to do MUni, I would skip the more freestyle oriented Nimbus, and get the Nimbus MUni.
There are a few people who have put the Nimbus Venture cranks through some pretty serious tests, and they have come out pretty good. They aren’t as strong as KH Moments, but the are a lot lighter, and still pretty strong.
I have become a real fan of the Felt “Berm Master” tire as a great all purpose trail/pavement tire. If you plan on getting a more street oriented tire I can’t recommend it enough. I realized that I have put more MUni hours on it by far than on my Intense DH. As a result my Intense looks brand new, and my Berm Master is holding its own on the trails. It would be annoying to have to change my tire whenever I wanted to take the uni off the pavement, which would be pretty much true for the Hook Worm.