I wound up with an S2 for my work, as it fit the bill much better than Phase or Hasselblad, but that's just for me. There is no general right answer to these questions, for some a camera like the S2 will make a lot more sense, for others, the Phase. For example, if I were going to shoot jewelry, I would say get a phase back and a view camera. That would be a much better solution than the Leica in the sense that you would have full perspective controls, macro as close as you want, and some sort of live view. If you wanted a travel or landscape camera, I would say the S2 (at least for me) -- the lenses are extremely good, it is weather sealed, the battery lasts forever and it is nicer ergonomically (to me, anyway).
One important consideration if you are using this for your own enjoyment and recreation is how it fits in to your workflow. Since you already have an M9, you will have no trouble adapting to the S2 files. They are quite similar to those of the M9, only with greater resolution and dynamic range. If you already use Lightroom, you are up and running with no difficulty. The S2 files tend to be spectacular right out of the camera, so there is not much fuss getting them to where you want to be.
If you are not already used to Capture One, it will take you a bit to get into the swing of it. Then you can either start using it for your M9 and 5D2 files as well, or you will need to operate separate workflows for each different camera. Personally, I am very happy that I can stick to Lightroom! I find it a much more appealing editor, and it integrates natively with photoshop. Just one less thing to worry about.
Overall, I have been very impressed with the S2. The lenses are the best I have ever used in any system, bar none (and I have pretty much used them all...literally...Canon, Nikon, Leica, Contax, Zeiss (Hasselblad, ZM, ZF, Rollei 6000), Schneider (Rollei 6000, LF), Fuji, Konica, Mamiya (Mamiya 7) etc etc). I have never seen a combination of speed, sharpness, image character (bokeh, je ne sais quoi) and freedom from aberrations like I have seen from the S lenes...they are big and heavy, but the least of your worries in any shooting situation. This is not to say that the Phase lenses are bad, very far from it. I don't think they will hold you back at all, but I also don't think they have that same knock your socks of effect either. The camera body itself is very nicely laid out in terms of the shutter release, grip, viewfinder and screen. I have to admit I dislike the 4 button interface...would have preferred it to be the same as the M9. But with the custom functions it is easy to set it up so that you can quickly change the things you need to change, and otherwise it just disappears in the hand.
On an image quality standpoint, I run a custom printing service, and the S2 essentially maxes out the printer. I use an Epson 9900, and at 1mX1.5m it still looks great. I am sure 80mp would give you more fine detail, but it is already so good, that I imagine there are very few times where this increase in detail would make a real difference in the end result. Again, your conclusions might be different. For me, the lenses, ergonomics and overall experience of using the S2 means I would not trade it for an IQ 180 and phase set up.
