I'd say to use both methods, of course. However, affiliates want to be assured they're communicating with a real person and not some spambot or scam artist.
Now if you want to send a weekly newsletter, a blurb, or just a pep talk, then the PSA Mailer is fine. But if you really want your PSAs to get started or learn the ins and outs of SFI, then contact them personally via email.