I'm leaning more toward writing none. Who is going to read it anyway? In my case, that would be one, a maximum of two persons.
That one person knows about my reasons. If I reasoned again over my rational suicide, then this letter would be too long and too short at the same time. From the outside, though, this person tells me I can go on – as objectively, I have a chance. I can't because taking that chance, even if it succeeded, would not give me peace at all. Conveying that state of mind and the reasons behind it is very difficult for me.
If anything, I consider printing out my musings on this site (anonymised, of course). This would include the journey and probably conveys more than a suicide note ever could.
I want other people for whom I have become only a distant memory not even to know I'm gone.