I've owned an RB67 Pro S, RB67 Pro SD, RZ67 Pro, and RZ67 Pro II. I prefer the mechanical RB but have nothing against the RZ that being said.
If you get an RB67 Pro that is the first model and it does not have interlocks to keep you from double exposing or firing the shutter with the dark slide in.
The RB67 Pro S has interlocks so that you cannot fire the body with the darkslide inserted and you cannot double expose unintentionally both the Pro and Pro S use foam seals in the back, in my experience it doesn't seem to matter if the seels between the body and back go bad but if the seals in the back fail you will get light leaks. I have never used the early odd shaped RB lenses just the later C and K/L lenses.
The RB67 Pro SD added the new K/L lenses I prefer the 90mm and 127mm in both the C and K/L configuration, the 50mm is also great. The Pro SD backs do not have foam seals so you don't have to worry about replacing those they also have a slot to store the dark slide on the back as opposed to the side of the camera body, I always used a Pro SD back on my Pro S. If I remember correctly the K/L lenses can be used on the earlier Pro and Pro S bodies but to use them on the Pro SD you need a trim ring that originally came with the lens. I believe it's only cosmetic and covers a gap between the rear of the lens and front of the body on the Pro SD.
If going RB I would suggest a Pro SD or a Pro S with a Pro SD back and the 90mm or 127mm for regular shooting, get a 50mm for the wide shots and if you want to do macro work with the 90mm or 127mm grab the #1 and #2 extension tubes. The 50mm will close focus to about 1.5" without the extension tubes and is therefore incompatible with them.
The RZ67 Pro is the first model RZ and also has no foam light seals in the back. The advantages of the Pro II are some improvements to the electronics, half step shutter speeds, and a fine focus knob. The RZ67 Pro IID has and integrated interface for communicating with digital backs.
If I were going with an RZ67 today I'd get a plain Pro with a 110mm F=2.8, and the 50mm. A few years ago when a Pro was $350 and the Pro II was $500 I'd say spend the extra on a Pro II for the fine focus and half stop shutter speeds. But now that Pro models are $1000 and Pro II's can go for $1800-$2000 I'd recommend the Pro over the Pro II unless you really need half stop shutter speeds and the fine focus knob.
Here's some examples of how much prices have increased on RB's and RZ's recently I don't know where they're going but hopefully someday they come back down. I'd really like to have another RB67 with the 90mm someday. It was my main camera for almost two years, I carried it everywhere and shot it handheld 95% of the time.
I bought my RB67 Pro S with a 120 Pro SD back and 90mm C lens in 2013 for $300, I've seen this kit going for over $600 lately.
I bought my RB67 Pro SD a year later with the same setup but a K/L 90mm for $450, I've seen these going for $800-$1000 recently.
I bought my RZ67 Pro with 120 back and 110/2.8 in 2016 for $300, this seems to be a $1000 kit nowadays with the 110/2.8 that was $150 4 years ago now going for $600-$700 alone.
and finally I got my RZ67 Pro II with a 120 back and 110/2.8 in 2019 for $850, these seem to go for $1800-$2000 nowadays.
The Pro IID? I have no idea the last one I saw was in 2015 ish and the body alone was $2100!