I am not a realtor, but I know both areas you are interested in (you have great taste- they are both beautiful!) I moved here 8 months ago, and have been renting in Southbay since then, but looked at homes to buy in both Southbay and South Creek.
It depends on what you want. SB is much larger- just over 300 homes. SB is not gated, SC is gated at night. SB has a private, and very affordable, marina-- I do not think SC does. SB has lots going on--several different groups activities, clubs, events, etc. I haven't participated in any yet- but nice to have as an option. As you drive through SB you will see lots of people out walking dogs, running, riding bikes, etc. Lots of children too. SB used to be exclusively an older population, but not any more. SC seems to be much more quiet, sleepy. They may indeed have social activities but its more secluded and quiet- just a different vibe. (Still beautiful though.)
SB stands out to me because of the activity, and also because of the clubhouse/marina area. I'm sure you've seen the pictures-- it's stunning. Clubhouse has a large room upstairs you can rent, updated kitchen area for all to use downstairs, card/TV room, lending library, gym, sauna, etc. Fantastic pool area. Really, really nice. I know that SC does not have a community pool, and I'm pretty sure there's no clubhouse either.
I went to a yard sale in SC a few weeks ago and spoke with a woman who is very happy there. But at my second stop, I met a man who joked that no one knows anyone-- they all keep to themselves.
Personally, I preferred Southbay... Incredible neighbors, amazing new friends. We just closed on our house a few weeks ago.
Hope this helped! Good luck to you!