Edit: Pay no attention, the following answer is mostly wrong… Read down
Different manufacturers. Corny kegs were mostly used by Pepsi. Firestone were prefered by Coke. The Corny kegs have ball locks and are generally more popular and a little more expensive (because supplies are running out and these are what many people have). Both work just fine, but you’ll want to pick one style and stick with it for your homebrewery.
Firestone kegs are also called pin locks - although since ball locks are more popular some companies are replacing the posts with ball lock posts.
Other differences - Corny kegs have a pressure relief valve in the lid. On pin lock you must press the gas valve.
Pin lock kegs are slightly shorter and wider than corny kegs.
My LHBS owner loves pin lock kegs because the gas and liquid posts are easier to distinguish. On ball locks they are different diameters, but very close.
Given the prices of kegs nowadays, it’s probably less expensive to put threaded fittings on all your lines and buy whichever you can get for less at the time.
There are lots of ‘converted’ ball locks on the internet. I’ve seen pallets of them at homebrew shops, but that’s a recent response to the short supply of ball lock kegs. I’m not sure about ones purchased 5 or more years ago.
I think you mean that these are ball locks, but the posts have detail differences.
For ball locks, you need to know that there were several manufactures, Firestone (I know I have some of those), Spartanberg, Cornelius and John Wood (never seen those). There are at least 3 different posts designs for corny kegs, Northern Brewer calls those A,B,C type. Williams Brewing also has a breakdown by manufacturer.
Jeff is correct. There are different manufacturers of ball locks.
The posts will be different in the threading as it attaches to the keg, the poppets are different, the pressure relief valves are also different in some cases.
The posts have different overall heights, but the ball lock fittings are all the same.
Nowadays, with the universal poppet, some of the differences are less important. But, if you take them all apart at once you need to get the right posts and poppets back to the right kegs or they don’t fit and seal correctly. I know from experience.
Converted pin-locks will be a different height and width from ball lock kegs. I believe shorter and squatter.
One of my posts, which I’m assuming is Firestone has 12 points. My other keg looks to be cornelius. But the pressure relief valves are the same, as are the lids. Maybe I have keg that was converted and still using firestone posts. I really don’t understand this stuff very well.
I have both. I like the positive feel and easy identification of pin lock posts, but the ball lock kegs are more universally accepted and are taller and thinner so more fit into my freezer.
If you know other homebrewers that keg, you might want to use what they use. If you don’t, then it doesn’t matter much. Pin locks are usually much cheaper. All of the brand new kegs being sold are ball lock and eventually that may be all that is available. If you get threaded disconnects though, it isn’t hard to swap them out.