As I write this, I am printing out a farm area mailing that I try to do every six weeks. Why six weeks? Because on an alternate six week cycle, I mail out a different set of letters. It is pretty expensive time/cost wise to do that many letters. Yes, it is a numbers game.
They say... 1) Postcards are easier to get glanced at and tossed. 2) Letters have a greater chance of getting opened and read. 3) Hand written notes and envelopes get opened even more.
You could spend about a dollar per letter mailed doing it yourself, or you could spend $2.50 or more each for a company to do it for you and maybe have "mechanized handwriting." I don't have the budget for the latter, so I do it all myself. I have a letter, tweak it a little each mailing, print letters and envelopes, hand sign every one of them, fold, stuff, and stamp all myself.
It's a numbers game and takes a while to play before you get results... unless you hit an owner at just the right time, that is.
Some people alternate between postcards and letters. Ultimately, if you have to pay for the card production, do you save any money?
What it comes down to is personal preference and budget. Choose a method. Decide whether you want total control and do everything yourself, or if you want to pay for someone else to do it for you. You can always experiment with what you may be comfortable with to see what works best.
Like @Shawn Parsh said, if you have other people looking for the type of property you want to invest in, reward them for their effort. A REALTOR® can do a lot for you. If you find one that thinks "investor," Team up with them.