"It saddens me" is a phrase most commonly used by mature or adult speakers or writers. It is generally considered slightly juvenile to use the latter phrase and that format, though, yes, they are technically interchangeable.
Following on from Diabolique's point: because "It saddens me" is usually associated with with mature speakers, it is usually associated with more serious and far reaching topics, so one is more likely to say, "It saddens me to see the state of the environment today" than "It saddens me to miss the party"
However, this is more of a common practice than a hard-and-fast rule, so they can be used interchangeably.
Although I have no idea what a 'juvenile speaker' is, (at the risk of sounding like one) I would venture to say that usage of the individual terms would depend a lot on the context and on the audience to which it was being presented. For example, I see nothing wrong with saying, It makes me feel sad to see the way the hurricane victims are suffering! I guess that's why one of the forum rules is that inquiries should be presented within a context instead of just presenting vague inquires such as:
Are these interchangeable? I waited awhile. I waited for a while.