Your Audigy will do the 5.1 decoding, so either player will be fine.
If you want to bump up to DTS, use the PowerDVD 4 software. I prefer it to the ATI player anyway. It finally supports the hardware assist from your ATI video card. The PowerDVD does software decoding for both 5.1 and DTS. I have a soundcard that doesn't have a built in decoder (your Audigy does). I added 5.1 speakers and the PowerDVD decoding sounds awesome using the software decoder.
I think the ATI player has little better picture, but the difference is small. The feature-rich PowerDVD makes the switch worthwhile.