If you truly are an original score songwriter then the best arranger would be one where every single parameter is customizable, and in my opinion that may be Korgs. I don't know why a pure songwriter would even want an arranger. Whatever you get will be canned output. A better option would be a synth workstation with sampling options paired with a computer. Arrangers are best when you want simple, repetative compositions-scratchpad, not for vast, complex compositions. I find arrangers to actually hinder the creative process-for me.
-just my opinion