HI torger,
UHHH. This is a really good question. I mean the first one the: Rodenstock Apo-Sironar Digital. This is cheaper, isn't it?
Is there a difference between the two with a 40Mpix Back?
The Rodenstock Apo-Sironar Digital is cheaper yes. I've owned it for my 33 megapixel back but I was not pleased with the corner sharpness especially when shifted so I sold it and replaced it with a Schneider Digitar 35mm which performs much better.
The Digaron-S 35mm is AFAIK even sharper than the Schneider Digitar, but has a bit too small image circle for my needs. To answer the question specifically - yes there is a clear visible difference in corner sharpness when pixel peeping between the two lenses on a 40 megapixel back. If your sensor is 44x33 rather than 49x37 it may be a bit more forgiving in terms of shift, but the pixels are on the other hand smaller.
Many of the Apo-Sironar Digital lenses are sharp, but the 35mm is a bit weaker. Officially from Rodenstock the whole Apo-Sironar series is intended for 9um sensors, ie 22 megapixel backs. It's a bit simplified though. I'd say that you might be disappointed with the 35mm also on a 22 megapixel back, while some of the longer will work nicely with 60 megapixel backs.
I've heard others be disappointed by the optical performance of the Sironar Digital 35mm (and pleased with other focal lengths), so it's quite clear that it's not one of the stronger focal lengths in that series.
About price difference, the Digaron-S is about €2300+VAT and the Apo-Sironar Digital €1600+VAT, so in percent it's a large difference, but if you add upp the RM3Di lens mount the difference does not seem that large. If your sensor is 44x33 40 megapixel the Digaron-S will be a much better match by far.