I don't think that there can ever be a formula for this, nor is there any right or wrong, you just have to do what sounds good to your ears. I think a much more important thing is how you play it, rather than the instrument you choose. The touch and phrasing should sound smooth and natural and be right for that instrument. Phrasing in particular is huge, you should try to hear the song in your head as if its being sung, rather than slavishly following the notes and their timing on the staff. good players never sound like they are in a rush to get the notes in, they let it flow.
Mike