I would have to say that 98 is faster (generally). Espcially when it comes to simulators. Reason being that 98 allows direct access to the hardware... of course it is far less stable than 2000 and XP.

Windows XP is a major upgrade from Windows 98 in the home market, providing improved stability, increased performance and a series of extras designed to help home and small-business users more easily ...

If you have any interest in retro-computing, you know it can be difficult to round up the last official bug fixes and updates available for early Internet-era versions of Windows like 95, 98, and NT 4 ...