I've used most 21's that have been on the market in the last 50 years, and all of Leica's. I have 5 currently. The poorest of the Leica's was the pre-asph Elmarit, followed by the first SA.
The Zeiss 21 is a lot better, and the 21 CV Skopar is as well.
The first Elmarit was produced to allow metering. I had it for a while for the convenience, but generally used the f/3.4 SA for its performance superiority. The only area in which the Elmarit bettered the SA was with respect to vignetting. The moment the ASPH became available, I got that and was astounded by the superiority of its performance.
Henning