The question is only about the case, when both conjunctions are used as a conditional (leaving the possibility to use "wenn" as temporal aside).
Let's start off with an example:
"Falls ich jemand anders wäre, würde ich woanders wohnen."
While the same sentence with "wenn" sounds fine to me, this one doesn't. Somehow I feel in case of "falls" the condition must be actually fulfillable.
What I believe:
"wenn" - works even for fantasized conditions
"falls" - the condition must really be an option at the time of the phrase
Am I wrong?
I would really appreciate acknowledged and reliable sources in both cases.