Well, I bought the Zeiss 135/2 and the 105/2.5 AIS the same day, and I came similar conclusions... then again, the Zeiss has a bigger (ans stellar) full aperture, is nearly APO, has somewhat better bokeh, and does close-ups.
But when in Portugal last year, the 105/2.5 (AI in the latter case) did 40 % of the shots, the 50/2 AI 42%, and the remaining 18% were done with the Tamron 45/1.8 (mostly night shots)...