Ok, here's my rambling steam of consciousness about this:
The reason for the 150/180 vs 300 comparison is basically that I do like to use a longer focal length at times for landscape photography. My preference for that work is my Alpa but obviously there are limits in terms of long lenses available for that platform. Actually I could also consider longer lens up to the Schneider APO-Digitar 210mm (the 250 APO-Tele-Xenar would be of less use to me overall). The 150/180mm lenses would also be a reasonable way to round out the Alpa for other situations too. My gut feel, yet to be tested, is that these lenses would be good candidates for cropping from if I need to without too much loss of optical vs digital resolution (compared to my Mamiya glass at least). Also the 150/180 do have reasonable image circles to leverage them for stitching for panoramics or rise/fall if needed plus also some level of telephoto effect. I'd rather go longer actually but that's not ideal with the Alpa.
I'm inclined to invest more in the Alpa because that is my MF digital landscape platform of choice. Truthfully, I'm actually a bit of a reluctant 645DF shooter - there, I said it
- but there are times when I do want to use my P40+ on a DSLR platform. It's certainly preferable to shoot with just the one camera system out in the field and hence why I was looking for some field feedback about the longer APO-Digitars. If the feedback was that these were hard to use, not as sharp as the wider lenses that I already own, etc etc, then maybe I'd think twice about pursuing them further.
Why the Mamiya 300mm APO? I can see how that might seem like a strange alternative. Well, this would give me telephoto depth flattening I'm looking for but at the expense of having to use my DF body (which I'd have to bring along for those situations too). I already have 150/210 focal lengths but the 300mm APO would round these out (actually I do have a MF 300/5.6 already but I am fond of the 300 APO as I've had it before). Certainly a straight 150/180mm comparison would point to consideration of the 150 D, or perhaps the 75-150mm, as I know that these are razor sharp lenses and very highly regarded.
Ultimately I need to test out one of the 150 or 180mm Schneider lenses, with my preference being the 180mm. I have the option of trying the 120mm APO-Digitar macro but that's not really a like for like comparison for one of the longer APO-Digitars. It might give me a better feel for the handling & DoF / focusing though.