for the sake of $20, buy the audigy 2 zs, you wont be dissapointed
Although I have never owned a Creative sound card prior to my X-Fi, I say go with the 2 ZS as well. I know a few people that have the Audigy SE and/or the 2 ZS and those that have both or just the 2 ZS love it so much more then the Audigy SE.
Just looking that the tech specs on Creative.com, the 2 ZS looks far more powerful then the SE.
So, for the sake of staying under your $50 limit, I say the 2 ZS.... But imo, the card with the best band-for-buck value is the XtremeMusic X-Fi but that one is quite a bit more then the 2 ZS so I understand