Are both words interchangeable? Rikai-chan defines them both as "baby, infant". So what would be the difference?
In many contexts they are interchangeable, but strictly speaking 赤ん坊 is the actual word for baby, and 赤ちゃん is like a cute, pet name derived from it.
Also, 赤ん坊 is the spoken (colloquial) form - see sawa's comment for versions that may be more appropriate for formal writing.