I wouldnt do soundcloud links unless they request soundcloud links. An email is a little more professional imo. And its a good idea to give them a brief description too. And not in the subject. Dont be that guy.
Never, ever, send tracks in an e-mail!! It will only get people pissed off! The labels like SC-links that have DL enabled. That way they can do a quick listen without downloading, and then download the track in full if it grabs their attention. If you don´t use SC, then give a link to dropbox, wetransfer or similar.
The brief description part is a good idea.
