Hi there,
I went through the same problem last month.
I had ALMOST convinced myself to get a Cambo WRS 5000 with a IQ260.
And then I visited a dealer here and had a prowl on his computer and man, the ISO35 (of IQ280) with good technique - the files just blew me away.
Two small observations that I would like to share from my experience so far (disclaimer: I have only seen a few prints and not made any of my own) :
1. As mentioned above, ISO 35 with good technique, the IQ280 blows away anything that I have seen until now in my 3 months or so of contemplating MF photogear. Even 8x10. The detail is different, feels more real than the IQ260 on screen.
2. The ISO 100 and 200 with good technique are quite comparable to ISO140 files of IQ260 for long exposures. I found that over the few hundreds of files that I saw of long-exposures done with the 280, this held true almost every time. Of course the sensor kind of caps out at 95 secs of exposure where it starts to show artefacts regardless of the ISO used, having said that, there is a wide choice available for exposure permutations below this time limit; I found that IQ260 files at ISO140 at a few minutes and a similar scene with ISO 200 at 95secs on the IQ280 were quite comparable with the sliders were tweaked, with little loss in dynamic range and noise levels.
On the other hand, I did notice a difference between ISO 35 and ISO 50 respectively of the two backs at short exposures. YMMV, but I suppose this would not make a great deal of difference either ways when it comes to printing even largish.
All in all, this suited me well. I don’t think I will ever need exposures longer than this considering the photography that I do.
So for the ISO35 available only on the IQ280, I decided for it.
Hope that helps.
thanks