I played with the 21mm and have been considering buying it for a few months, gotta get rid of the 16-35mm first. I think it's an absolutely incredible lens. Build quality is amazing, ridiculously sharp edge to edge, excellent contrast and color rendition. I think its awesome, I dont know anything about the Tokina but I doubt it could be any better than the Zeiss.
It's not really a fair comparison, it's a $700 zoom vs. an $1800 prime so its pretty clear that the Zeiss is going to be better.