Johnny Guest,
I would place IMI (TZ & TZZ headstamps) on the heavy end of the spectrum. I just loaded up a bunch of .45 brass and there was some TZZ mixed in. A lot of people don't like to load it because they have to remove the primer crimp, but that's not a problem for me.
Speer recently changed the way they make brass, eliminating one step in the process, which in the long run saves them quite a bit of money and time. They also occasionally have outside vendors make some of their brass. I know for certain that they had some of their 357 Sig brass made by an outside vendor, because I asked them at the SHOT Show if they did and they confirmed it. In fact, I'm supposed to send them some 357 Sig brass that has unusually small flash holes that they said they weren't aware of. I discovered it when I started decapping some with a Dillon Carbide 357 Sig die and the decapping pin wouldn't go through the flash hole.
Anyway, I would put Speer .45 brass in the middle of the group.
I form a lot of .45 acp brass into .400 Cor-Bon, and I've found that Winchester, Federal and Top Brass (made by Scharch Manufacturing) are the three brands that stand up the best. In fact, the Top Brass also comes in +P and has some of the most precise primer pockets and flash holes I've ever encountered in mass produced brass. You can tell it was made by a machinist.
I've got some nickel plated .45 brass that's been around awhile, too. I just prefer the brass cases, since nickel is a pretty hard substance and I don't like it when it peels off.
The Cowboy Action Shooters go through a lot of ammunition, so a lot of times they shoot what they can get their hands on. I don't know when nickel plating was started on cartridge cases, but it was very common on pistols in the late 19th century for rust prevention. The technology has been around for a long, long time.
I started loading in 1963, and there was a lot of nickel plated brass around then. In fact, the gun shop I used to buy brass at had two big "fishbowls" of .38 brass. One had small primer pockets and the other had large primer pockets and you got to choose which you wanted. I'll bet a lot of reloaders on this forum have never seen .38 special and .357 Magnum brass with large primer pockets.
Hope this helps.
Fred