Your are not crazy, it's a good idea. I have had it for a while too. I purchased 31ltd in January 09 and now year later I bought used 43 just last sunday. Both silver and with colored leather bags
Geee, can't wait until January 2011
I havent spent too much with 43 yet, so can't/won't comment much on it except that from f2.8 it seems ultrasharp. It even brings out details you sometimes don't want to see (think skin etc).
As far as PF goes on 31, in sub f4 you can get it if you try, but normaly it's not a big deal. In most situations it's actually well controled. What I find bit more annoying though is green(ish) CAs around just OOF strong highlights at wide open or very close. If you don't nail the focus pefectly you'll know it because of this, and sometimes it's ugly, IMO. Other than that, 31 is one SUPERB lens.
BR
Peter