2 way would be used if you want to both upload new files from your phone to dropbox and download new files in Dropbox to your phone. You can configure deletion and modification options to varying degrees of synching. So if you wanted to only sync new files, but not sync deletions, you could do that. 
It's pretty powerful - I'd recommend installing, reading the large help guide (linked to from the menu), and playing. 
What I can say for sure is that I've never seen it fail. You can even see both status and history of the syncs you set up.