noimmunity
scratch my niche
series of M & LTM lenses that I use with it and my M6.
It seems to me there are actually two different questions here. One is whether or not to buy the 50 summicron? The other is whether to keep the M9 or replace it with an XPro-1?
The answer to the first question really depends on what lenses you already have and what your needs are. There are a lot of excellent 50s that are cheaper than the current 50 summicron. Heck, the current 50 Summarit is arguably better in every way but aperture. Not to mention all the other possibilities...
Comparing the M9 and the XP-1 is like comparing the M9 and the D7/800. These are really just two very different camera systems, neither of which is a replacement for the other. That doesn't mean you would need both any more than it means one is definitively better than the other.
You can certainly get insurance for your expensive M9 and lenses.
My intuitive sense, without having seen any of the details about your lens line-up, is that the 50 summicron idea doesn't come from real need at all. To test that hypothesis, you could go and get one of the respected gaussian formula 50s and see how much you really need to use it.
If money is a concern, you will be much better off in the long run to put a cap on your M system. That means decoupling from the road map of expensive digital M upgrades and giving up the quest for 'completion' in the M system.
I think you ought to keep your M system but re-assess what you expect from it: forego the 50 cron, rethink a reasonable=limited lens line-up, decide if you need an M9 or can do with an M8 (or should cash out now in preparation for the M10)... Then make a well-considered strategic move into another system like the Fuji X-mount that appeals to you and that can be used in parallel with the M.