Frederick all the way. Much more versatile for one thing:
Fred,
Derick,
Eric ... and
Jacob has never appealed to me much as a character. Though I'll be hearing the entire sound track of '
Joseph and his Amazing Technicolor Dream Coat' in my head all day. Which is fine; I like the musical!
Where I live, if I saw a reference to a
Jacob person, I'd assume he pronounced it YAHkop.