I used to have my regulator set up with fittings for both when I used to go to club events. “Just in case” anybody ran out.
Years ago, we heard the stories that all the soda companies were switching from kegs to bag in a box. The thought then was that kegs were going to start going away and getting scarce. “Pin lock” were Coke kegs, called that because the fittings have little pins that stick out. “Ball Lock” were Pepsi kegs, I think there are little stainless balls inside the fittings.
I’ve been lucky enough to acquire 5 of the 3 gallon kegs over the years and they are what I primarily use. All ball lock. Every now and again they still show up on ebay for a decent price. Mine all have one welded handle. I never found this to be a problem. I still have a few 5 gallon kegs but I rarely use them. My kegerator is set up for homebrew kegs. I don’t go thorough it quickly so a couple 3 gallon batches lasts me awhile.
Brand new kegs are expensive. In the late 80s and into the 90s when I was acquiring all my kegs, used 5 gallon kegs could be had in great shape for $20 or $25. But the 3 gallon ones were still more money. Every once in a blue moon you would see the rarest of them all - the 10 gallon keg. Not many if any of those around any more. Most of them were cut up and converted into unitanks or fermenters.
How about the colors of the rubber tops on the 5 gallon kegs? Most of them are black. I have a green one which I think is also pretty common and a red one. I think the red colored ones are pretty rare - I haven’t seen them much. I think there are also yellow colored ones that I have seen very rarely.