You've got a point there. But I don't see any bad products that Terratec makes. Many of their sound cards are based around one major product, much like the other companies. I don't think you could go wrong with this.
The XFire is hardly a hype product, I only know of one online dealer in the US that you can even buy it, where as there are many places that sell the Santa Cruz(with all its acclaimed PC awards) sound card. Granted the XFire has PC awards too, but they're not high profile media grabbing ones, oh and they don't claim audiophile sound quality, lol.
The XFire and 6Fire are different cards in terms of technology, but they have similar features. I said it as a confidence booster.
Since you seem to have a very tight budget, I don't think I can help you decide which of the two to go for. My advise is look for GOOD reviews of both cards, considering you probably can't try them without first purchasing them.