Bootcamp vs OSX Client
Most users report better performance using Windows, expectable, since the OS X client is not a native application but a wrapped one.
Just cancel the installer when the big download starts and copy your current game files from the Windows partition over to
/Applications/Guild Wars 2.app/Contents/Resources/transgaming/c_drive/GW2
This way you can try it yourself without big trouble.
Ah thanks very much! Thats just what I needed! Cheers.