I haven't pointed it at the sky, but generally speaking the Zeiss 21mm 2.8 makes the canon 16-35 look like a coke bottle.
http://www.adorama.com/ZI2128DTZE.html
If you do need flexibility then the 16-35 is the ticket, however compared to the zeiss it needs to be stopped down significantly to be performing very well. The zeiss 21 is so sharp that in my wedding photography I've actually had to soften files with it for album printing.