I have yet to beat the main story for the first time. I own WM I but I haven't bought WM II yet. Without spoilers would the story flow better if I complete WM II before finishing the main story? I have completed most of what was in WM I if that makes any difference. 

The game ends when you finish the main story. You have to complete all side adventures, including White March, before the point of no return (you'll know when it happens). However, it's implied that some time passes between WM part 1 and 2, so you can take a break and do other stuff before you return to White March.

By what i saw, Finish the main story to the point of no return first... Or at least to the very end of chapter 2 before you go to WM2. Otherwise some secrets will be revealed early, Not to mention whats going on is a little more fitting scaled up in power. But some End of act 2 secrets are blown and one Act 3 secret is hinted at.