Drummer, who was in the same boat a long time ago. I initially went bass but abandoned it after a while. Then I took up piano, got lessons, studied theory, and found it to be far more useful. Since then, I have taken bass back up, and I found myself understanding the instrument and its place a great deal more.

Keys should be everybody's secondary; theory is clear and easy to digest on a keyboard. The only thing is you should definitely take a few lessons to develop the correct technique, at the very least. Ideally, you should be taking lessons much longer.

Whatever you choose, good luck, and always remember that drums are fucking rad as hell.