I'm currently running on my xifi prelude optical out to an av/receiver supporting Dolby digital.
I'm wondering which card is better for optical out or are they the same?
Only difference if you're only going to use Dolby Digital or DTS via optical is the extra DSP effects. With the X-fi you can use Eax and CMSS while the HDAV gives you Dolby Headphone, Dolby Prologic 2x (if you have an old receiver that only has PL2 support). Sound quality wise the HDAV should give you pretty much exactly the same sound and DSP features as when you use Audio Creation mode on the Prelude. This is if you're only going to use optical with Dolby Digital and DTS.
The advantages of the HDAV (I have both the Prelude and HDAV 1.3 Deluxe btw) is much higher quality analogue outputs, HDMI out and support for Dolby True HD and DTS Master HD via analogue and HDMI if you watch blu rays. If you really are only going to use Dolby Digital and/or DTS via optical to the receiver there isn't much of a point in going to the HDAV.