Since this post has been resurrected after 8 years (!), I'll give my opinion as someone who has owned both from new. I bought the RD-1 in 2004, the M8 in 2007. Both long gone now...
The RD-1 is the better camera if 6 MP is sufficient (you don't print large) or don't use ISOs over 800. Its design is far more pleasing and analogue than the M8. However, it is more fragile in build quality - the rangefinder mechanism is especially prone to going out of whack. On the plus side, you can easily dismantle and repair/service the camera - if you're mechanically minded. The M8 - as we know now - is unreliable too, in different ways (sensor and shutter, especially). Both are well past their sell by dates with unavailable components, so don't expect too much help from the makers!
In hindsight, I wish I'd kept my RD-1, and toy with the idea of getting another one. But the 6 MP is deal breaker - I need pin-sharp A3 (20 inch) prints, which are impossible from 6 MP.
Of all the digital cameras I've owned or used (20 or so), the RD-1 is the only one I truly took pleasure in using. The others were mere tools.
I will never buy another Leica digital camera having experience of the M8 and M9. Totally overpriced, unreliable and underwhelming performance. And ridiculously lengthy repair times - I waited months!
I currently use a Nikon D800E for its resolution and reliability (since 2012). Great images but I have no affection for the camera at all - I find it unergonomic with way too many bells and whistles, and think of it simply as tool like a screwdriver or hammer.