In the quick looks I've made at each language, I found Prolog much easier to understand than Erlang. Prolog's pure logic syntax is simple if you already know 1st order expressions. Erlang borrows some syntax from Prolog but doesn't have Prolog's simplicity.
Erlang has its roots in prolog (was originally built on top of prolog, actually..) Software Engineering Radio's episode 89 is a great listen for anyone interested in Erlang
