The professional version has a field called "To:" so that someone can take messages for other people. The personal version assumes you are taking messages for yourself.
Both versions are GPL'd; in fact they are exactly the same source files (though the interface files differ). The personal version is free. The professional version costs one billion times the cost of the personal version.
No both use the same file format. The personal version actually just fills in the To field with an empty string. Also, these are both completely compatible with version 1.0 of the software.
I guess, but it does lead to confusion as to what program should open a given file. I recommend using one or the other.
I'm so sorry to have added to your misery. I'd strongly recommend the following print resources: