The 1DsM3 has had enough resolution for my needs and my clients’ needs. That said, I am always driven to give my clients the very best that I can artistically and technically. Now, when shooting architecture using the new tilt/shift lenses, it’s comforting to know it’s not a compromise. That’s not to say there aren’t differences between a DSLR and a medium format digital back, there are. But I think that these new lenses, used on a 1DsM3 or a 5DM2, even the playing field quite a bit. They make the choice less about technical issues and more about other issues – your personal shooting style, subject matter, ease of use (gear & software), on location workflow, post-processing/post-production needs, your clients’ needs & budgets, ability to have a backup kit on site, etc.

I think if you follow the links above to Rainer’s findings you can extrapolate from there. It has direct tests between a 5DM2 and a Sinar back. The Sinar back would be similar to any Phase back with a similar sized chip in it (33mp.) The difference would be greater if the chip is higher res., like with a P65, or less if the chip is of lower res. Beyond that you would need to factor in the things I just mentioned (and ideally rent each setup to really know for sure.)