It doesn't sound like you are short of the cash if you are already looking at a WATE, but really any Leica lens makes a great landscape lens on the M9 (my favourites are 50 and 90).
But if you maybe think you'll only use such a wide lens every now and again the CV 12mm and 15mm lenses could be considered. You will get the 'red edge' effect whatever you code tham as, but using Cornerfix software will sort that out. It means for less than £1000 you can get two very wide lenses with a good progression from 12, to 15, to your 21 and then whatever else you have. While the differences are noticable I have always thought the 16-18-21 progression tight, meaning you could just as well walk nearer or further away rather than need a different lens to do the job.
Steve