The mark of a great 12 inch version is where it's at least as good as, and preferably better than, the original single.
In the case of Smalltown Boy, the original was the 12 inch, with the 7 inch a poorly edited substitute. The longer track starts slowly, with just voice and electric piano, plaintively singing in a way you can really hear the hurt in Jimmy Bronski's voice. It builds to where the single comes in, but that's a couple of minutes after the track began. It's a Bona fide classic and well worth the price of admission alone.
Others have mentioned the Mari Wilson track, which is nice, but a typically stretched out version of the single, with an extended instrumental beginning before it loops round to the first verse.
The power station track has some great backwards forwards stuff in the intro too.
All in all, for the current price of under £8 it's well worth the wedge!