Hello! I’ve used both of these and I have a SoundblasterX G5 myself, which has the same surround system as your Z.
Stick with the SoundBlaster!
I’ve recently reviewed the Cloud II dongle because many people have asked for a full article on it. It doesn’t show up as a 7.1 device in Windows, and so it doesn’t get fed any surround data from games…only stereo.
Your Soundblaster card is a true surround device in Windows, and as such will make proper use of the full surround audio coming out of games.
Hope that helps you out!