Hi
According to my experience, you should have individuals projects.
Here are some reasons:
- you keep different level of conversation
- subtasks are not so easy to use: see To Subtask or not to Subtask
- the My Tasks of the colalborators will be much clearer with individuals projects
- you can use the Progress Update Status, you would lose it with one big project
I guess there are other reasons but these are the first I think of.