Well, I guess I'll weigh in here.
IMO, these comparisons are very difficult to sort out. Owners tend to hype their choices ... as well they should since they DID make a decision to get one camera over the other (AKA: purchase justification). I guess I can speak with a little bit less purchase justification, since I purchased ALL of them :ROTFL: 2 M8s & 7 lenses (mostly fast aperture ASPHs), 2 Sony A900s and all of the Zeiss optics + the 70/200/2.8G, 1 Nikon D3 and a .... D3X ... plus most of the modern AFS, nano-nano lenses including the 200/2VR. My primary purchase justification is that I make my living with this stuff, and do a lot of different types of photography. For me, "Horses for courses" simply means I have a stable of horses (the other barn is packed with the BIG MFD horses which ARE the IQ KINGS).
If the image quality of the M8 is to your liking, then a claim that it's "old sensor technology" is meaningless isn't it? IF an improved M digital rolls out in the next year or so, rest assured it will NOT be in the Sony price range like used M8s currently are. Plus, NO ONE knows if and when a M9 might be making its debut. "Bird in the hand," and all that.
While the Sony A900 body is a nice size, the Zeiss lenses are BIG and heavy ... there are no smaller Zeiss W/A primes yet, so for wide angle work it's the two Zooms ... which are not small. I can just about fit an A900 with the Zeiss 24-70 and a flash into a bag that holds my entire M system.
WHO IS KING of 35mm sized IQ? :wtf: Depends on what combination of what camera, what lenses, what ISO, what shooting conditions. The A900 with the 135/1.8 is great. The Nikon D3X with the 200/2VR is greater IMO. A900 with Zeiss 16-36/2.8 is fab, the Leica M8 with 24/1.4 and 28/2 is faster and more fab ... and the Nikon D3 with 14-24 is Fab in its own right because it shoots circles around the Sony and Leica at ISOs above 1000.
My opinion based on your criteria? If you don't have any Leica M lenses, just get the Sony and a couple of lenses. In general most people are more comfortable with the diversity offered by a DSLR that allows you to expand your horizons. If you have a M8/9 or whatever, one thing isn't going to change ... the focal length spread is 16mm to 90mm, no true macro, no long lenses, no possibility of T/S.
On the other hand, IF you are a rangefinder person, and love the way you think with one, then there is no substitute. Do get wrapped up in the logo on the Leica, think about the way you ... think creatively.
-Marc