There was some discussion about this over on ARC. One poster suggested that the difference between them is just the consistency. He believes that they are all the same 'grain' or particle size, and the higher numbers (1200/1500) are just 'thinner' than the others. His suggestion was to buy the 500, and thin it down to whatever consistency you needed - this way you got the most product for your $.
Personally, I have used the 1000 as a primer, and it worked very well. You have to thin it with lacquer thinner, and it needs to be thinned quite a bit. If you don't, you'll get the dreaded spider-web effect as it shoots out of your airbrush. The thinner evaporates quite quickly, so don't let it sit in your AB too long...
Take it for what it's worth....
Murray