I have both, and both have advantages, though my ultimate preference would be for the M6TTL. On the R3a, the 1:1 viewfinder is quite nice to use, particularly on the long end, say 75mm. The patch is large and the viewfinder presents the 75mm framelines exclusively, no pairs (in fact, all the framelines exclusively). It also has aperture priority, which came in handy last week when I was shooting a bullfight in Madrid, and a faster top shutter speed, also handy if you like working wide open in a lot of light as I do.
The build quality of the Leica is superior, however: I think it's always useful to have a seemingly benign tool that can, in a pinch, double as a weapon. Loading is much faster and easier -- fewer missed shots in the field for me -- and the shutter is significantly quieter. I wear glasses and can comfortably get down to 35mm in the viewfinder (.72 version), 28 if I absolutely had to.
Between these two, it's a bit of a toss-up if the prices are similar. If you have a shot at an M7, however, that would be my choice.