2-3 Zone works great against teams with poor outside shooting. You can really shut down an inside game with a - position 2-3 zone.
3-2 Zone works great against teams that shoot a lot of 3's, especially if you have a rebounding edge inside.
I also use the 3-2 in late game lead situations.
Doubleteams also seem to be more effective in zone than M2M in my experience, I'm not sure why.
On the other hand, man to man is significantly better than zone at rebounding, so I'd use the m2m if you are at a disadvantage in RB.
Also, I think if you have good defenders, M2M is a better all-around defense, especially against balanced attacks.