I have both of these cameras myself. One thing I will mention is that the M8.2, at least by my standards, does well at 1250. Sure there might be a little noise in the shadows, but nothing I would consider a problem. So I would say the M8.2 is not completely out of the running. And the fast lenses partly make up for the ISO issue.
Now here's a reason to bring the D700. First, no high-ISO worries. And then, you can put a zoom on it. That will eliminate having to change lenses (and risk dropping them) with frozen hands. Of course, by that time of day, when High-ISO is needed, Sue and I are more interested in food, and can be found in that great Russian restaurant a few blocks from the Holiday Inn.
Personally, I would leave the D700 home, and take my chances with the M8.2. I would also bring my Fuji X10, which has great Hi-ISO performance, and is smaller and lighter than either of the others!
You know, Tom A's wife, Tulikki, knows loads about Helsinki. Maybe you could PM Tom for some suggestions!
Enjoy your trip!