The reason being, is that although I won't be hosting on Codeplex, they support Mercurial and there are plenty of tools that .Net developers can use to bind to the SCM and the fact that it is a distribute system means it wins in my eyes. I will need decent branching support and I have never been a fan of SVN's branching mechanism.
I am willing to be educated more if you have any compelling reasons for me not to go with Subversion.