To a great extent it's horses for courses.
When Win 98 came out the clock speed of your average PC was 233MHz (I'm being generous - it was more like 166 or 200 MHz)
So if you're still running an 'old' chip PC, XP ain't gonna do much for you. And I mean PII, probably PIII, or AMD K6 II and III going up to Thunderbird, Duron.
But if you've got a nice little P4 or Athlon jobbie running at 2-3 GHz , do you think it would be a good idea to run Win 98 on it?
Remember the architecture of your chip wasn't even around when Win 98 came out. XP was designed specifically for the architecture of your chip - yes Win98 will run it, but can you expect 'full' performance? I have to doubt it.
In my own case, I've installed and run Win98, ME, Win2000 Pro and XP Pro all on the same architecture (Athlon XP from 1500+ to presently 2000+). I can tell you that the Win 2000 and XP Pro versions CONSIDERABLY outperform the earlier versions of Windows. I'm not surprised.