I'm taking my 645 to Nelson Ford this week. I happened upon another 645 and a 5906 in the last week.
I am planning on the 645 as a carry gun, but should I keep the second 645 for it's back up? I also acquired a 5906. I could use this as a back up gun to the 645.
Now I don't want to have a trigger job done on a gun that is "merchandise" for obvious reasons.
I don't like the short grip on the 4516 so that model is out. The size and weight question is irrelivant to me; I frequently carry two Smith N frame revolvers.
The other 645 would offer magazine interchangeability, but the 5906 has the high capacity magazine.
I don't like little guns as backups as I think that in a desperate situation, your primary has malfuncitoned or you got injured and lost it, you really don't need a tiny gun that is much harder to shoot accurately than your primary.
So there is the quandry: two identical guns or two different guns but the same manual of operations.