I am always amazed at these discussions comparing S2 with H4D. Surprisingly, you find no photo comparisons - though many are fast to point out they own/use both. Many seen to be talented photographers with great galleries to prove it... Do they really need the tiniest, almost undistinguishable differences in IQ - that would probably swing either way on any given situation??? I have an H4D31 and always wonder what the H4D41 would really give me in terms of solid IQ advantage. I also would love to try out an S2, because it would seem more of an outdoor camera - which is what I wish to concentrate on. I have lots to learn from people like you - but don't seem to get much scientific evidence to help me on my decisions to upgrade/change.
Actually, I've posted some direct comparison shots including large crops ... but they were more about focusing than pixel level Image Quality. I haven't had the S2P long enough to make any declarations in pure IQ comparisons to my H4D/40 ... it takes awhile to zero in on processing techniques with a new camera ... where I've used the H4D/40 for a much longer time.
Image Quality and Image Qualities are two related but different evaluations IMO. One is more quantifiable and the other more subjective. The two get mixed up quite frequently.
IMO, the differences between the H4D/40 and Leica S2 are much more about the physical properties of the camera itself as it relates to the photographer's needs. Any minor differences in IQ most likely will play second fiddle to these considerations. If a versatile modular camera with more advanced AF for off-center compositions, a vast array of accessories and lenses including a T/S adapter, and a much more effective tethered option and a proprietary, highly tuned software is what the photographer needs then the H4D/40 is worth a look. If 35mm DSLR type handling, more mobility, less versatility, lenses that require less proprietary software corrections, and superior weather sealing better fits the photographer's specific needs, than the S2 is worth a look.
Personally, I've decided to employ the Playtex Bra strategy of
"Lift and Separate", and decided to up the H4D kit to a H4D/60 ... which will win the IQ contest hands down ... so when needed it is there with all its accessories (like a waist level finder and T/S unit), versatility (like use on a view camera and superior tethered use in studio), and vast lens choices. When all that clutter is not needed, the S2P will be the camera of choice. However, not all photographers need this much versatility for the range of work they do ... where for others it is more about
Horses For Courses.
-Marc