I've bought and sold the dedicated Kirk plate PZ-35 (and the RRS version) for the Leica M so many times, that on the rare occasion that I need to use a base plate, I just use the generic one that came with the ballhead (I have a BH-3). You don't really need a dedicated plate unless you're a frequent and fanatic user. I've had no problems with the generic plate, used with the heaviest M lenses (Noct, 90 APO, 135/4). Just make sure everything is screwed on tight. I keep the camera strap around my neck, only because I don't want to risk the tripod toppling over with the camera and lens on it.
These dedicated accessories are sometimes just more dead weight to carry around. I can use the generic ones on any camera. The only dedicated L bracket I have is for the DSLR; I think I used it twice.
http://www.kirkphoto.com/Camera_Plate_for_M6.html
FYI, Kirk has an eBay shop where you can sometimes get deals on plates.
Best of luck with your decision!