Does it matter between the two? ...I figure SSN is easier just because most people have it memorized no?
I think that may be the difference: individuals use SSN and businesses use tax id (correct me if I'm wrong)
I'm always worried someone has set up a site that looks like an affiliate but it's really a phishing page that's just really too good to be true : ( Sometimes you can wait to enter your SSN, and that can be preferable or something to watch for from the security end. Best of luck!