Both are about the same price at the moment. One is compact and of limited capability while the other is huge, faster and is presumably one of the best ever for 35mm format.
Though I decided on the APO Sonnar 135/2, a while back, I am still not sure because of the size.
I easily can live with a lens that takes 49mm filters.
This is for use with an A7R. The M lens is of limited use on my MM requiring an additional VF and such hassles. I am not in a rush as I am not in desperate need for a 135mm lens.
Anyone has any input?