My opinion, which is based largely on second hand info. Creative makes the better overall hardware, but their drivers tend to be garbage, which can often hamper the experience. Turtle Beach cards may not be quite as good hardware wise, but the drivers are generally higher quality.
That being said, I would DEFINITELY go with a system that connects directly to the sound card if we're talking about some kind of surround sound signal being transmitted. Those can take up a lot of bandwidth, and so I'd personally rather not take the chance it gets backed up somewhere on the USB bus.
And there's a post by some guy over in the Computer Help forum who has an issue with a USB sound card you might be interested in reading. But the long and short of it is, is that the USB bus is really a pretty poor choice for a sound card if you do much more than listen to MP3s with it.
I have three computers with various Creative sound cards. My main computer has a X-Fi Audio Extreme card. I believe most pc gamers are familiar with these cards. How about Turtle Beach sound cards, or those supplied with USB headphones? I have never tried anything but Creative.
In my opinion, the Creative onboard sound card along with quality 5.1 desktop speakers is best. However, I'm forced to play using headphones to block out other pc sound systems and/or Xbox 360. Turtle Beach headphones come with splitters so as to keep both desk top speakers and headphones plugged in together. They also provide quick disconnect so as to unplug the headphones without reaching to the back of the computer. Why bother with any of this if USB 5.1 headphones are available? Can the tiny sound card supplied with USB headphones compete with onboard cards in terms of quality and sound? Anyone?