I would keep the 15mm 2.8 which must be PERFECT for astrophotography (if this is your thing of course).
That's partly why I bought one.
For wide angle landscape, I don't see the need of AF. Plus it is wider than the 16-35 IS.
+1. As a landscape lens, manual focus is fine.
The 15mm is a special lens for me. It might only get used once or twice a month, but when I need it for those UWA shots, it does it's stuff really well. And at the risk of being burnt at the stake, AF is a little over rated in a lens if the subject doesn't move (eg: landscape). For me, landscape is all about the sharpness, contrast and colour rendition, which the Zeiss does really well.