Well, everyone can play Flash video, but it can’t be exported out of FCP without 3rd party apps.
Quicktime will play on PCs that install the Quicktime drivers. Most people can do this, but some can’t – due to unwillingness, lack of computer savvy, or strict corporate firewalls.
Bear in mind that Quicktime on a PC isn’t the friendliest bit of software – constantly advising to upgrade to QT Pro, wanting to become the default media player, etc.
So, to be as accommodating as possible, Flash video is the way. But, its entirely reasonable to expect a PC user to be able to play a QT file.
(or you could make an mpeg-1. yuk!)