Wow. Very different lenses. My 21 is much less often in my camera bag since I got a 28, but my 28 is f2.8 and my 21 is f4.5. I'm not sure the slow 28 f5.6 is any substitute for such a fast 21 as you have. I want one of these new Summarons myself, but I might just have to continue wanting: it is an indulgent addition and no substitute for anything I don't think.
PS. So January is really not the month for such reckless decisions. Who knows what might follow?