I agree with the above. The multiple grouped role plays often have more interesting interactions. However, the down side is, it's usually one vs the other. Which in the end is pretty much over done no matter how you look at it. As I said before, though, it usually brings up better role play experiences to have others to role play with outside of your group.
Single grouped role plays are more difficult to role play. Due to the fact that people often seem limited as there isn't another group present to bother or defend against. Most role plays don't have single groups [talking about mapped role plays here] as it can be more tedious to think of a good enough plot to keep interest. However, the good thing about the group is recruiting, you only have one group to recruit for and look after. Which can decrease the stress factor by a lot, not to mention members are more close knit, and tend to focus on role playing friendships and bonds, as well as possibly participating in some form of mysterious plot that may come about in their own group.