This is just an opinion -- not from direct experience on this question.  Personally, I receive a lot more emails than I do physical things in the mail... so I think you might catch more eyes with a mailing.  A downside is people not being able to click on a link from it to register right then if they want to.  (But, making it clear on the mailing how to register could help.  You could even consider creating a QR code if you wanted.)  And, if doing a mailing, it could be followed up with an email to all those people a few weeks later.