There is going to be zero productivity gain for you.
I work on both Mac and Windows, and I would say this statement is probably close to the truth.
However, it's not black and white:
- I love working on Mac, it makes me feel good. I really miss expose, the dock and double-click-to-hide windows when I'm on Windows now.
- I hate working on Vista, the new explorer frustrates me and the aero window borders are not easy on the eyes. The basic theme is even more distracting. Turning themse off doesn't work anymore because the explorer windows need themes for their toolbars.
- I like working on XP, it is fast, stable and I know it well.
On Windows I mostly use gVim, Notepad++, Netbeans, Ruby (from the console), Subversion and Sqlite to do my cross platform Java and Ruby stuff. The reason I stick with Windows is because I earn most of my money writing .NET and SQL Server applications.
TortoiseSVN on Windows is magic. I miss that when I'm on the Mac.
TextMate is great, but I tend to go for either vim or Netbeans.
I use Debian Linux to do any and all server related jobs, like hosting repositories, running Trac, that kind of stuff.
Sorry to ramble, it's just that this is something I'm quite interested in myself.
P.S. Macromedia/Adbove creative apps seem to feel much more at home on the Mac. Flash's toolbars and especially the ActionScript editor suddenly make sense. On Windows they feel kind of bodged. I think CS3 improves matters though.