I haven't used the 75 Heliar, but I've used the Apo-Lanthar 90mm on both the M6 and the R2A. I'm no lens testing expert, but I found it delivers superb image quality. Along with the Heliar 50mm f/3.5, it's one of my favorite lenses. I'm not sure how to describe it, but I find the color rendition very accurate and pleasing. Close in this aspect to the rendition of the Pentax 77mm limited f/1.8, another superb lens for portraits.
The 90 is difficult to shoot with. If you're used to streetshooting with wide-angles, switching to a 90 will bring a lot of motion blur in your pics at first. (Obviously).
To reach the max res, you need some form of rest, or a lot of sunlight.
I would not recommend low light photography with this lens, it's just too challenging for the photographer.
Also, using the little 90 frame in the M6 or Bessa's finder is adding to the challenge. If possible, use a separate view finder or a magnifying correction on the VF. Your framing and focusing will be much improved.
On the upside, along with pure image quality, I found my subjects were less intimidated because of the longer distance between you and them. This adds a lot if you like to portray people with a natural pose.
I plan to try the 90 on the E-p1 or e-p2 once I can get my hands on one of these.
With the in-body image stabilisation, that should help a lot...