The resolution of the M9 is 29% greater than the M8's, using the formula square root of the difference in sensor size (in megapixels).
On topic: I bought an M9 (after owning a couple of M8's) before I began to move into Leica lenses from the ZM's. I suggest to buy what makes sense to buy first. Glass, if you think the 50 asph is for you. M9, if you think you'll benefit from its improvements over the M8 more than you'll enjoy the 50 asph. Looking back, I think I'd actually buy the lens first because the 50 asph is really really good for much of what I shoot, which includes dark clubs etc.
I have made a few prints of equal size and dpi from both cameras (up to 16x20), of the same scenes shot under similar light with the same lens. The differences are much less than I expected. Slightly better micro detail and native color from the M8, slightly better shadow detail and tonal range from the M9, but nothing that set me confident that I'd done well to sell my M8 and buy the M9 on the strength of imaging potential alone.
There are reasons to buy an M9, like manual lens correction and full frame benefits, but I don't think image quality is one of them, unless you make really large prints or shoot above iso 1,000 a lot of the time.
On topic: I bought an M9 (after owning a couple of M8's) before I began to move into Leica lenses from the ZM's. I suggest to buy what makes sense to buy first. Glass, if you think the 50 asph is for you. M9, if you think you'll benefit from its improvements over the M8 more than you'll enjoy the 50 asph. Looking back, I think I'd actually buy the lens first because the 50 asph is really really good for much of what I shoot, which includes dark clubs etc.
I have made a few prints of equal size and dpi from both cameras (up to 16x20), of the same scenes shot under similar light with the same lens. The differences are much less than I expected. Slightly better micro detail and native color from the M8, slightly better shadow detail and tonal range from the M9, but nothing that set me confident that I'd done well to sell my M8 and buy the M9 on the strength of imaging potential alone.
There are reasons to buy an M9, like manual lens correction and full frame benefits, but I don't think image quality is one of them, unless you make really large prints or shoot above iso 1,000 a lot of the time.