Postcards VS Letters

[Jacob Joseph]

Hi, I am about to start Direct mail for the first time, I am wondering what you would prefer postcards or letters when sending out mailers. I Know postcards are much cheaper than letters, and I know mailers in general is a volume game. I am also curious when creating your mailer template what is the key part you focus on to make your mailer unique compared to everyone else sending mailers?  

Also, side note I invest and live in the Metro Detroit market which is very competitive.