I do both, and personally, for projects I like working with a group I know and trust.
That is not to say that working by yourself can't be done with great results but I'm just more of a people person myself.
You can brainstorm and work with ideas and come up with new viewpoints in a group. On your own, you're stuck with your own experience - which right now I'm finding tough because I am working on a project on my own this very minute and have hit a "designer's block" slump.
Sure, you can call all the shots when you're by yourself, but sometimes it's good to receive direction from a supervisor you know and trust the opinion of.
I find working by myself is bad for my health (staying up until 4 or sometimes 8 in the morning, drinking red-bull, missing family activities...)
There are pros and cons to working in both. But, in the end, I like working in a group better than on my own because of the faster work-flow and overall feeling of morale when it's all done. I mean, when you finish a movie by yourself, who are you going to party with? :)
Think of what the mixing stage must have been like when you had Erik Aadahl and Ethan Van Der Ryn in there at the same time (Transformers)... Must have been awesome and fun to work with them.. I can only dream.... The fun those guys must have. Some of the stages I have seen have basketball hoops and ping-pong tables for breaks. It must be a blast.
Oh yeah and here's a great article I almost forgot to post a link to:
It's about sound design teams and how they work together.