Neither.

For XP you want a PCIe X-Fi Titanium and for 7 a Z.

Thanks for the suggestion but as I said, I've just recently acquired them so I don't really want to spend any more money. Especially since these are brand new and sealed. My i7 870 Windows 7 Ultimate rig also doesn't have a spare PCIe slot unfortunately.

The Audigy 2 ZS or 4 is better suited for your P3/Win9x machine IMHO.
I agree with Mau1wurf that XP era and above the newer cards offer more of a advantage, but as you already have them I'd just put one in and test for a while, if you don't like it drop it in the P3

My Audigy 2 Platinum PRO works perfectly under windows 8.1 .
Difference between Audigy 4 Platinum PRO is 0. They are exactly the same and have same sound quality/creative shit.

The only thing this card is not suited is motherboards with out PCI slot. Operating system makes no difference. And using such a powerful audio processor on an crappy Pentium 3 with Windows 98 would be a waste 😀

I don't think you should compare the two since pretty much the cap of sound quality was reached a long time ago. The only thing that can be improved is CPU offload and different sound color OP-Amps. Than again, all of that is pretty much pointless because you can Digital OUT into an professional sound processor and get the sound you want.

Plus, you have external console with Audigy 4 PRO, which is additional comfort.

Ok, well thanks guys for your advice. Guess I'll put the Audigy 2 ZS into my Pentium 3 Win98 rig and the Audigy 4 Pro into my Pentium 4 Windows XP rig. I'll leave the Asus Xonar DS where it is in my Win7 rig.