I have sold my 5DMkII and bought a second 5DMkIII. No regrets. Now both of my cameras are the same model so no thoughts in choosing.I have been thinking about doing that for a while, but there are four reasons I haven't:
In addition when you shoot with both cameras the controls are the same and this is a plus.
I would suggest to upgrade if you can afford it...
#1, I won't get much out of it
(Yes, I realize the prices will only keep falling, but losing $1,000 - 1,500 sucks)
#2, I have the EF-S screen in it and that works great with my fast lenses. The 5DIII viewfinder is a grainy, blurry, piece of junk in comparison and is all but useless for manual focus.
(I also realize that there are 3rd party solutions to this problem, but I'd rather not fool with them)
#3 I find myself shooting fewer events (and thus less need for two bodies with prime lenses on each one) and focusing more on shooting with a single body, so the 5DII has become more of a true back up body.
#4 I have spent way too much money on camera gear over the last few years and really need to get back to taking car of the house, cars, and other things like that
All your reasons are quite personal so quite reasonable too.
The only reservation I would have is about #1 where you said it yourself: "the prices will only keep falling".
By doing this exchange I saved about 40% of the price of 5D3 so I have no complaints myself