I've had both. For me, I kept the Santa Cruz mainly 'cause it was the most problem free of the two, but that was a while back. As to which to keep now, it really depends on a few things.
1. Do you really want the breakout box? Or is it giving you headaches like it did me (ie. USB not functioning half the time).
2. Driver issues. Hercules has come a long way in terms of drivers, but when I had the GTXP, Voyetra's drivers were much better. Future driver support for Voyetra is somewhat of a question mark though. Although they've recently released two beta's, they waited almost a year before responding to users from their previous release.
Also, the company has moved away from the consumer-level sound card business for the time being. Herc on the other hand, seems commited to continued driver support for their entire line of sound cards. Now that they've moved onto a unified driver platform for all their sound cards (with the exception of their CMI8738 based ones), expect more driver updates and improvements in the future.
3. Do you prefer to listen to music more so than playing games? This is assuming that you've got one of the newer Herc GTXP's that uses the same CS4624 DSP as the Fortissimo series. If it's an older one that uses the CS4630 DSP, same as the Santa Cruz, then this is a moot point. I do find that the "lower-end" CS4624's seem to handle 3D postional gaming much better than it's CS4630-based brother, where as the CS4630 has a better SNR and can reproduce music with much better clarity and crispness. If you ARE a gamer and your Herc uses the CS4624, then I'd stick with that.
