Wow, this question is from July and no replies. The main difference between the two cameras is that the RZ has some electronics that the RB doesn't. The RB is an entirely manual camera. The RZ takes batteries and the RZ lenses have electronic shutters. If the battery dies you can still use the RZ but you're limited to a single shutter speed of, I believe, 1/30.

The RZ is definitely the more versatile of the two. The RZ will take most, if not all, RB lenses but the reverse is not true. RB backs can be used on the RZ body, the reverse is not true. The RZ glass is generally considered slightly better than the RB but there are some fantastic RB lenses available. The KL lenses particularly.

I have the RZ Pro (not Pro II) and love it. Highly recommended.