98 is faster, xp is more stable. I wouldn't really use either. If I were to use windows for a server on a box that old, I'd use nt 4.0 probably, or maybe 2k. The 9x series is just so unstable and insecure.
linux would be faster and more stable than either though, it might be a better idea for an old rig like that if you do not absolutely need windows for some reason.
I'm running a file/print server on a k6-2 333 using linux. It's been up for a month or so now without any crashes (before that the power went out) and it doesn't waste resources on gui stuff. That tends to not matter on a fast box, but on a slow box, you'll really feel the cpu cycles you lose to the gui.