The older are compression formed
The older ones are compression formed ("CF"), and usually a dull finished red (but can be silver and other colors). If you look inside the case you will see no separate basewad, just the smooth curve left by compression forming.
The new ones are grey, and are assembled from a basewad (that is curved to match the internal contour of the CF case), and a straight tube. These are referred to as "High Strength."
There is talk about the basewads in early High Strength cases coming free, migrating down the barrel and being loose in the bore. This would constitute an obstruction and would cause a problem the next shot.
I have so many of the old CF cases I've used very few of the HS cases, and had no problem with them.
What I use, more than anything else lately, is any of the Remington Compression Formed cases, even the cheap black ones. Most Skeet and Trap ranges have them lying everywhere, free to pick up. Beware, there are some Remington brand cases that are built up with a flat base wad that are actually European and very different. Results and data between the various Remington CF cases are nearly indistinguishable. It's hard to see any difference in data and results between the Remington CF cases and the old Winchester CF AA cases.