Never used the 35mm 2.5, but have nothing but good reports on it. I used to have the 35mm 1.7 but sold it recently for the 35mm 1.4....both are brilliant. I have an M2 with the 50mm Summicron, but that is the only Leica lens that I own for RF, the rest are Voigtlanders. Every one that I have performs really well, with the 75mm Heliar and the 21mm Skopar being standouts.
Congrats on the M6, they are rather special.