I have 2 3-9x 40's, one with the standard z plex one with the rapid z 600. It appears to me that the rapid z is a little thicker where the hairs cross and at last light is easier to see to me than the z plex, but the z plex seems to draw my eye to the center quicker because of the darker thick section of the crosshairs.
I have had a couple 3x9x40's as well. The rapid -z cluttered the sight picture in my opinion. I just could not get used to it. I thought the standard zplex worked better in low light situations. The rapid z did OK. If this is what you are going to go with I would suggest the 50mm objective lense. My buddy has this one with the larger objective and it is much better. Just my opinion though.
The new version of the RapidZ is much less cluttered. I just looked at it this week at the SHOT show. I did not like the old one at all, and the 2013 version is nice. They are even working on a simpler version for the near future on the "Terra" line of scopes.