I made the mistake of getting the other kind of 4x6 paper. In my opinion, it's not strong enough to go as postcards.
In the 39 or so Postcard Exchanges I participated in I had great results using standard Ilford RC paper, stiffened with Avery style laser labels that covered a significant percentage of the back of each card.
I expect the inkjet labels would work as well.
I used a template file and the mail merge facility in my word processor to put names, addresses and accompanying information on each card, but if you prefer to hand write the cards instead, the labels accept ink much better than the RC paper does.
FWIW, the old Ilford Postcard paper was specifically identified as being on Portfolio stock.