I was going to say I liked your first design best, or just stick with the "old" network cards.
But after seeing that Sti one, I think something like that is a great idea! Postcard size is bigger, and with a full color picture taking up the whole card it would definitely draw attention to it. I've had the small business cards left on my wipers before advertising various things and not even realized it until I was driving and saw it flapping because with the wipers in the down position you can't see them all that well from outside the car.
I also like the big bold letters complimenting the owner's car.
If I walked out and saw the message "Nice SVX!" I'd subconsciously be that much more interested in what it had to say.
Not to "borrow" too much from the Sti one, but I really like the layout with the big colorful picture and minimal information. And of course the place for the member to put their name is a cool touch too.
Only problem I see with postcards is to do them right they definitely need to be professionally made. To get the crisp photo printing and the sorta "laminated" paper stock postcards are usually made of (for stiffness and semi-weatherproofness). If we could get in on some kind of freebie deal like that local group did, this would be awesome! Otherwise, it would have to be an expense to be worked out somehow and might be best to stick with what we've done.
Although, for $150 you get 5,000 DOUBLE sided cards, so if we did want to get wordy we'd have the whole extra side to do whatever we want with.