I own both the system and since Street photography is what i enjoyed doing, here is my personal review on the suitability and claims of these cameras for this genre.
Instead of going thru the specs of each system, i will just cut to the chase.
M9 - what users claim as the ultimate street photography tool
- small size and not intrusive like a typical Canon/Nikon DSLR
- mostly coupled with fast 50mm or 35mm and the discreet shutter mode
- full frame and 18 mp allows generous cropping and high IQ
GXR - what users claim as very suitable for street photography tool
- 28 mm at 2.5 lens
- small compact size
- APS-C sensor and 12 mp has good IQ and high ISO
Do note that Street photography i am referring to going close, say 2-5 meters away from the subject and shooting it. Those who shoot using zoom lens is not street photography, its voyeurism. I had the same misconception once and thought i was doing great street photography using the Canon SX30IS, after looking at much images i realized that its different and missing the main ingredient of street photography, getting close.
The other factor to take note is that, unless you are shooting from the hip, Street photography will be very limited. Shooting from the hip allows near zero visual hint that you are aiming a shot and hence minimizes any posing shots.
The main reason of using M9 in street photography as you can see in various Leica Forum and popular sites like Steve Huff Photo site boils down to it being discreet and the ability to cock shutter at a later time after taking a shot and smaller than DSLR size.
The GXR totally surpasses the M9 in this main point. The leaf shutter on the GXR is so quiet that snapping it like a distance of 1 meter away in a normal environment and even a bookstore is easy. The LCD on the GXR can be turned off so that it doesn't even appear to anyone near or behind u that you are shooting anything.
I find the M9 shutter loud in comparison and the discreet mode of cocking the shutter later is only suitable requires further effort in masquerading (you need to pretend to turn and face something else then release the shutter). The GXR could take multiple shot discreetly given the same time. The only thing you need to set on the GXR is "snap focus" and select 5 meter (or 3 if u prefer much closer shots).
When it comes to size and weight, the GXR is significantly lighter and smaller.
What the M9 is better is the ability to have significant faster lens and the full-frame allows much better cropping and IQ. So i would think that if you want high quality shots, the M9 will deliver them and when you are not doing 100% street photography cause u saw something that you can aim at and shoot, the M9 gives you the shot that you can be happy with.
The other advantage of M9 is the ability to change distance/zone quickly in the event you notice something else interesting and you want to quickly capture the shot.
So in conclusion :
GXR is the ultimate street photography tool, if you are looking at discreet and size/form being the main factor.
M9 is decent in street photography but performs much better in IQ in any situation.
The other camera that could be of interest are the Sony SLT (DSLR replacement) and NEX. If you handled a Sony SLT apsc, you notice one thing, that its very light and comparatively smaller than a Canikon, so coupled with a 35 mm 1.8 lens, it could do wonders in Street photography since the shutter is also very quiet. In fact its more quiet than the M9.
Updated : jafleming3 (dpreview, leica forum) pointed out that X1 is an even better tool if one were to consider package sized and discreet. The claim is that X1 can set manual focus to 3 or 5 m and the LCD turned off while shooting from the hip. So there you go.
There is also now the M-Module A12 for GXR. Early users report indicates that not only you can use any M-mount lens but the shutter is SUPER quiet. X1 vs GXR ? That i will leave to those who are lucky enough to own both the system to provide comparison.
Updated : Harold66 (dpreview nick) have offered other views in support of M9. You can find his posts here : http://forums.dpreview.com/forums/readflat.asp?forum=1013&message=39433937
Sample images of shooting from the hip :
Here is a shot i took using M9 today, from the hip at F2.0 and 5 meter. The ISO is high though cause this is indoor.
Also on the M9, but using F/1.7, shot from the hip, indoor.
Here are some using GXR, 5m snap focus.