Miniature painters are fiercely loyal to their primer. Many will only use Games Workshop primer, and we go through about 6-8 cans of black each week. They will put off painting rather than buy another brand. Armory is another choice, often because it's inexpensive. Loyalty to Armory is about being cost conscious, for the most part. My preference is Testors/Floquil primer, but I'll grab GW primer as a second choice or Armory in a pinch. Right now I'm priming white with Floquil, Black with GW and gray with Armory.
My current issue with primer is availability. Testors was making their primer available through the game trade for a while, but they pulled out. I heard they were concerned with alienating their hobby accounts by supplying product to the game trade. My new source for Testors is an Internet discounter. I wait for primer to go on sale and buy a bunch. It's then marked up a small amount from there. It will always be cheaper elsewhere, like Hobbytown, but in a pinch, we try to have it. Black refuses to go on sale, so for new we have white and gray.
Armory is actually a house brand of Alliance Game Distributors. This the biggest game distributor in the country, but not our primary distributor. Lately they haven't played well with others, or some such story, so my main distributor no longer carries Armory. I would very much like to wean customers off Armory, but I know it will be an uphill battle.
Our replacement for Armory is the new house brand from ACD Distribution, called Board to Pieces. Reports from ACD are favorable and we've got sample bottles for our painting pros to check out. It's a little more expensive than Armory, and all four brands have different formulas, so these aren't all direct comparisons.
As for availability, Games Workshop has the highest "fill rate" of anyone I order from, so there's almost always primer available to order. However, as orders arrive only once a week, sometimes product is available, but in transit, which doesn't help you as a customer. Armory is now harder for us to carry, but we're not giving up on it. However, sometimes it can be gone for weeks at a time. Board to Pieces will be the easiest primer to carry, with next day delivery when we're out of stock. My solution is to carry each of these primers deeper, rather than annoy customers with out-of-stocks.
If anyone wants to do a head-to-head write-up, I'll provide the free primer (but you have to do the write-up). Heck, I'll even reimburse you for a can of Krylon if you want to throw that in the mix.