Various versions of this discussion have been had here before. If you were comparing to the non-ULD version of the 50, I would say go with the 65, which I adore. But the 50 ULD is very special. If the ULD is in your budget, I'd say go for it. As someone with 3 RBs, I got an RZ specifically for the 50 uld and the 110/2.8. All my other needs are met very well with the RB system.
Personally, my interests tend toward longer lenses rather than wider. To me, the perfect RZ kit is 65 / 110 / 210. But if you favour more environmental shots and might want to do more cropping then you may well want to go wider.
Nothing to apologize for from either of these lenses- they're both superb.
Note that you can pick up the latest version of the RB 65 in great condition for next to nothing... and it is also a superb lens. You'd merely have to choose the times on the lens rather than on the RZ body, that's all.