Which method is better for a complain letter?

I agree with you. If I were to make a complain letter, I would not write an email and send it to the person. To me it is like sending a non serious letter because the number of emails we receive everyday is a lot. As for me, I have many email accounts and in one account I receive at least 50 emails per day. I can't check all my emails and I will delete the ones which I think are not important of spams. So there is a huge possibility the complain letter will be deleted by the receiver if you send it by email. Other than that, by sending a formal letter using the postal service, the letter will arrive on the table of the receiver and it can be seen and touched. It has more personal touch in it and the receiver will automatically regard it as something serious. The chances of the letter being left unopened is almost zero percent.

Nowadays, eletronic communication is faster than the usual brick and mortar. This is due to the fact that it can readily be passed on to the management. While letter can still give value or sort of, however, letter can be misplaced and it might loss on the way. Emails are better. And when you write an email, see to it that you write in such a way you writer formal letters. In this way, your email has formal tone which your intended recipient could not just deny. Emails are good as far as convenience is concern.

I think that a hand written letter would be much more effective. It looks like you've taken the time you write it and you really do have a problem with something.