Dave, you may not like my recommendation...
Worrying over which M-body to purchase for a back-up secondary is a pleasant position to be in. We are a spoiled lot when we wring our hands over the benefits between the beautiful MP or M7 zeitgeist. But for practical considerations, the issue to me would be a change-up in viewfinder magnification than the body.
I had thought about this dilemma for quite some time, before I decided on an M7 with .58 viewfinder, married to the M3 with the .92 patch. Having two bodies with one dedicated to wide(r) lenses, while the other carries the longer, just seems to me to be more important than worrying about whether my battery fails, or which colour body has which film in it.
Keep in mind too, that for your wedding needs, the M3 can now focus a 135mm easier than a .85 body, let alone your .72.
Of course, this means selling your .72 (if you want only two bodies), which is why you may not like what I say above! 😉
(Will you be at MOCA tonight for the Contact Photo kick-off party? If so, let's meet.)