I've had various people tell me different things in regards to postcards over letters.
People say they prefer postcards over letters because they are something that can be looked at quickly to see if there is any interest. Letters have to be opened and read, and if to wordy a person can lose interest quickly.
And then some people say that letters are better, because you can include more information and appear more professional. Especially if addressed to a specific person in the business.
I personally prefer the postcards. I think you can add more of your own personality with just short, quick pieces of information about who you are and what you do. I've also gotten a better response from them.
Its also been suggested that you should hand-write the addresses instead of using labels, that it appears more personal.
I'm sure others will have other opinions but thats mine
and Lily I like your postcard with the business card as return address, nice touch!
And yes do include your phone number!