Part of SNSD's appeal are their routines. Some of them like the Genie one or the Mr. Taxi one are almost as well known as the songs themselves. Because there are 9 members each of them can bring their own "character" to the group.
They are very good for audience participation on their tours. On live TV shows they focus more on the performance than shouting to the audience, it's what most idols seem to do. I find them a lot more genuine than 2NE1.
I think CL's fake urban attitude is so annoying. It's like she's always putting on an act and it comes across as so fake and prepared. I like 2NE1's group personality, but I find them really fake and staged compared to GG.