is a feature comparison between the two. In my opinion, phpBB3 is better in every way.
And yes, you can upgrade from 2 to 3 without losing any posts or users.
If you have not installed either yet, it would not be wise to install phpBB2. It reached end of life last month. It is no longer supported by the community here or the developers. There will be no more security updates for it, etc.