The SK 28mm f/2.8 is a discontinued lens and I believe was never a big hit to begin with. I was even told by another photographer that some of Schneider's sales staff discouraged people from buying it. Point being, the only way to find it would be to get an used version, which could be difficult.
The new 28mm I hear is very good and has a 90mm IC. I have thought that maybe the lens would be a worth wild purchase, but with the CF, you would be looking at $7500.
I mainly shoot interiors with a 35mm and find it to be more than enough to tell the story. About 30% of the time, I really wish I had a 43mm as well. Occasionally I find that I want a little bit more in the image, but I just can not justify spending $7500 for a lens that I only would use a few times a year. I'd rather put that towards a back with a larger sensor a couple of years done the line, which would solve that problem (and others).
I'd say you can not go wrong with the SK 35mm, just make sure you get the CF.
Correction: $7500 price tag includes the cost of mount in the Arca R mount. For you, it would be more like $6500 (assuming you went with a recessed board).
Closer to $5,600 - ish with CF and lens board (copal shutter).
The older 28mm as Joe mentions never caught on too well with digital due to the small image circle (60mm), although I do have a few clients that picked it up for use with older, smaller chipped backs as a low cost workable solution.
The difference between what John Paul is asking - the 28mm and the 55mm, can come down to capturing with 1 shot what might take 2-3 shots with the 55mm. But also, even in 1 shot, you're pretty limited to what you can do with tilt or shift (rise/fall). A better "budget" solution might be the SK 35mm Digitar, with a 90mm circle, as long as you're not using a 60/80 megapixel digital back.