Guesstimate: Message passing implies a fully self-encompassed message carrying enough meaning for potential distribution between systems? [Substitute: Erlang VMs, physical machines, or whatever] Function call typically means within the same interpreter instance / language virtual machine / operating system native process.

(ie. in cases where parallelism exists or is warranted, a message passing model enables it. In cases where it does not, they are essentially the same, possibly message passing being slightly slower due to marshalling/encode/decode overheads. These days that overhead is usually irrelevant and thus message-orientation is a plus, as it encourages smaller components and thereby helps to bring clearer architectural paradigms to complex systems)