I would find snailmail postmarked by the American military post office really exotic! 
But when I read “Is location of postmark important to you?” I thought of something totally different! 
Since I take my snailmail for special postmarks once or twice a week I always leave space on the cards for those postmarks and ask for "please cancel to the left/to the right/in the corner/… of the stamp. So the postmark does not touch too much of the stamp. 
an apo postmark does not list the location. only the zip code. apo are temporary mobile post offices which can move at any moment. that does not mean all apo are mobile. american military bases abroad use the apo adddress, no mention of location. here is a picture i found of one i have but can you guess the location - it is a base in germany. of the sad situation in afghanistan, the apo bases there closed and the zip code does not exist until the reassign to another unit/base when needed. i hope that give some answers.
must add that you can find where in the world is the postmark is from as looking at the postmark, it has apo and 2 letters (if ae - means europe and aa means asia). hope that helps.
about prices mention above, only apo which would be cheaper than usa prices is germany. other apo - if looking for a cheaper price to send postcards, i would send via apo than the local country rate. some might say “hey, my country has cheap postal rate”. but does the usa have a military base in that country? have a great day!
To me, postcard, stamp, and postmark are all important. The more interesting the combination, the better. But remember, what may be common for you may be rare for somebody else. Just pick what seems to fit best according to destinations or profiles, or follow your intuition …
