This is going to feel like a non-answer: but if you need to ask this question in this format, save yourself some great pain and use Postgres or MongoDB, doesn't really matter which, just something known and simple.

Normally you'd make a decision like this by figuring out what your peak demand is going to look like, what your latency requirements are, how distributed are the parties, how are you handling attachments, what social graph features will you offer, what's acceptable for message dropping, what is historical retention going to look like...[continues for 10 pages]

But if you don't have anything like that, just use something simple and ergonomic, and focus on getting your first few users. There's a long gap between when the simple choice will stop scaling and those first few users.