Email because you can send a scheduled email. That means you can write the email today for example but schedule it to be sent at 2pm, 8 days later. It's really up to you. What that does is, it enables you to change your mind if you want to. Or not have to worry about what people will think if your unsuccessful because then you can simply delete the scheduled email.
Lile I would put very personal things into the emails. Then when I was unsuccessful I didn't have to cope with the knowledge that people saw me at my worst in those emails, or knew my secrets for example if I named the relatives who sexually abused me. So it was like "yikes, I've failed again but now I can delete the scheduled emails" it wasn't "omg! I've been unsuccessful and everyone now has my emails and now knows everything" I prefer scheduled emails, at least then if I was successful they'd find me after the scheduled email was delivered. That could be days later realistically but they'd think I died on the date of the scheduled email of that makes sense. Not unless they were intelligent enough to realise it was a scheduled email x
Letters, you can hide them but there's always a change of someone finding them if your unsuccessful and then using them against you in the future to talk about your mental health. Whether that be someone genuinely trying to help you. Or someone who's abused you trying to use your mental health as a scapegoat whilst denying their abuse of you? Ecetera.
