For me as a recipient, it depends.  A random post card or envelope is likely to be tossed. One on higher quality stock or with a time-appropriate message may get read and even remembered before tossing. A personalized letter is mostly seen,  by me, as too desperate unless the addressor is known to me. I imagine the answer will vary by different factors. 

 Trade-sized postcard, large print.

 Or, handwritten address on a white envelope, with a first class mail commemorative stamp on it. 

It really depends Will on what is being sent ... the message, the reader's need/interest in the offering, etc. 

Post card

Post cards have better chance of getting a glance before throwing them in recycle bin.

it depends on what you are doping it for. I do handwritten thank you noteds every day and that is something for an envelope.

Although I love a good letter, we have short attention spans these days - I'd have to say post cards, Will!

Depends on the quality, as Ron and Alexandra Seigel stressed!

It depends. Usually a postcard gets a cursory look before hitting the circular file

Will,

I throw away postcards printed on junk paper, and those letters that comes from a mass mailing without reading the first and opening the second.  If it is well done, I pay attention...I happen to hate CHEAP, because, there are so many ways to do something ELEGANTLY that does not require lots of money. A

In my case it all depends on the return address on the envelope Will Hamm . I'm going to glance at a postcard but as we just witnessed with the huge onslaught of political postcards many only get a very brief glance.

A post card is more likely to get read, and much less expensive.

It is advertising that you are talking about. A quick glance will determine my next action. In the recycling or a more thorough review. Both often get discarded before being read. Some get my attention.

Will .... you know the answer to this so ????

They will read the post card as it is easier.

Depends what the purpose of the mailing is.

I often do not open envelopes from agents 

It's hard NOT to read a post card, since the message is out in the open.  Envelopes from unknown people get trashed.

Another vote for a post card. I generally rip and toss junk mail. I toss the post card but generally look at both sides of it since I don't have to take the time to open it. 

I think a postcard is more effective.  I tend to toss 'marketing' material in envelopes without opening them.

It depends.

I think they would be more likely to read a flashy postcard, unless the envelope is made to look like something personal, rather than an advertisement.

I for one throw both away without taking a second look

I think it depends on the presentation.

As a recipient, I almost always read the front and back of a postcard. Some envelopes just get ripped up and thrown away without being read (depends on the sender), and if I take out a letter from an envelope, I'll glance through it but won't read the entire thing unless it's very interesting. Which is rare.

Hands down, people will open an envelope.

Depends on your presentation.

More people will definately read the postcard, starting with the company that creates it, all of the people in the mail delivery process and of course the end recipient will have not choice but to see some portion of it even if they choose to not read it all.

Send a hand written note in an envelope.

Post cards always get glanced at, but a hand written envelope - I always open those!

Postcard that serves a purpose calls to me. The other stuff is old world

I really think post cards.

A hand written envelope gets more attention although once the sender's information becomes known the jig is up and postcards rule once again. You need to find the sweet spot of frequency and offer something for best results.

Post cards

Will ~ I use postcards whenever possible since they can see the message and I know I recycle many a letter addressed to our household without ever opening them.

If the 'Sender Address" has no authority, then the Hook on the post card will prove more effective.

EXCEPTION: if the envelop is invitation size, it will be opened and read.

Post Card, when i receive a letter i look at the post mark 1st....1st class possible opening....standard immediate trash

Agree with Michael Jacobs

a postcard, if there's a graphic that MAKES then hesitate for a moment and glance at it... it works for us...Tammy picked up another listing yesterday.... 

I think the envelope, but make sure it has a hand written address and maybe a colorful envelope.