Speed wise I don't think it will make a huge difference, maybe a little one but nothing more
Usally I would have the Desktop acting as the server, simply because it isn't moblie, if you close your iBook the network goes etc
Is there a sharing of an internet connection?
If your using a wireless network I find that the Mac is better, since it has encryption when setting up a network and sharing the internet, Windows doesn't seem to have this with its Internet Sharing, well I can't find it at least