Having attempted to get a Git server running on Windows using gitosis, I can attest that:
"hg serve"
is vastly more straightforward!
Update
Hopefully Smart HTTP Support in Git will make this more straightforward, but of course will depend on how it is made available if we are comparing it to the ease of "hg serve".
Update 2
If all you want to do is quickly share a repo with a colleague, then "hg serve" presents a beautifully straightforward way of doing that. Setting up a robust and secure mutli-user server is another matter for both platforms.
Update 3
This is a very decent start at hosting Git repositories using IIS with Smart HTTP Support. Works for me:
http://www.jeremyskinner.co.uk/2010/06/25/hosting-a-git-server-under-iis7-on-windows/