I would get the twins. They seem to have a longer play factor in that the bitty's head get scuffed and marked very easily. Also I have noticed that the hair is a nice touch because then it can be brushed by little ones and decorated.
My almost 2 year old has a bitty and doesn't like her hard head and we also have a bitty that I rewigged into a baby Ariel and she loves styling her hair as well as sleeping with her.
I think at that age it's the hair for softness and styling that is really entertaining.
You could always by a bitty and rewig it in whatever color or style you want. It's very easy and the hair is much better then the twins. And then you don't have to worry about selling the one off.