Bonnie, I would say that there can often be a big difference between songwriting and song singing. Why would your friend recommend a songwriters course instead of a singing course? Probably because the friend sensed that you were so independently creative -- and not that you were needing necessarily to sing -- that you might as well add words. You strike me as someone who is already working away at being creative as an instrumentalist, and it wouldn't be that big a leap to start adding words as an added facet of that creativity (or not as it suits you). But that seems to me a different project than expanding your vocal range, interpreting songs, etc. They can be the same, but not necessarily -- not everyone is a "singer/songwriter" -- I personally have no interest in being a songwriter, but improving my singing voice would be nice.

yours, Peter T.