I guess this is all technical and at the end of the day a nicely captured shots with 1ds3 would be quite nice too and I know a few professional landscape photographers who sell their photos for a living use 5d2.
To Jim2:
This forum can get very heavy on Theory, but the last time I looked, it was named Photography, not Pixel Theory. (Maybe there should be another forum for that). So before you go drop thirty or forty grand on a MF system, based on Theory Feedback here, I'd advise you to call CaptureIntegration and RENT a system. It might be the cheapest money you ever spent.
Take your 1ds3 and that rental system, and GO SHOOT REAL PHOTOGRAPHS, and then come back and make real Epson prints, and then lay them out on a table, and do not get out a magnifying glass, (unless your potential clients do). Act like a normal human being, and simply look at the prints. Do not put on a Lab Coat. Just stand there and act like a real human being. Do not consult an MTF Chart. Just be a Photographer, and stand there and compare the prints.
Then ask yourself again the original question that you asked here, and then see if you think it's worth writing a check for forty grand.
Many of the so-called science experts here have never purchased a MF system, let alone used one hard, yet they seem quite content advising you on a sizable purchase. Theory is fine, but your potential clients aren't writing you a check for Theory; they're writing you a check for Ink On Paper.
Only if you do this can you truly answer this question for yourself.
And, if you do this test, make sure you bracket, so that you get within three stops.