I have an M6. I plan on keeping it, and later I will buy an M8. Here are the considerations which matter to me.
One film Leica is enough. I love film; I still love it more than digital.
***Most of all, film photography works better than anything else for the "decisive moment" of photography, right up through the moment the shutter is released.
***After the shutter is released, film is more trouble than digital, to process, to transmit, to reproduce.
The thing is, nearly everything I want to do after the picture is taken, is something digital.
If I want to stay with film, I must invest US$1,000+ for a scanner worthy of the negative, e.g, Nikon Coolscan, plus the continuing expense of $10-$15 per roll of film+processing. And I am told that most scanners do not like silver-halide black-and-white film, so I'll probably say goodbye to XXX and hello to C-41 "chromogenic" black-and-white (and probably Kodak rather than Ilford XP2, since scanners do not like black-and-white film without an orange mask).
I will probably do this while I save up for an M8.
My main concern with the M8 is this: I like the look of pictures taken with my M6 and my 35mm Summilux ASPH. On an M6, a 35mm lens is a wide-angle lens.
On the M8, my 35mm lens becomes, functionally, a 'normal' lens (as a 50mm would be on an M6). So if I want to make wide-angle pictures with the look I like, I shall have to get a suitable 28mm or 21mm lens ... to make images with the wide-angle look.
So, I'll keep my M6 and keep the 35 Summilux on it.
Eventually I'll get an M8 with some nice 21mm/28mm lens, three batteries, and a fistful of SD cards.
Hope these thoughts help --
A U S T I N ,
Flickr: Austin Burbridge's Photostream