As Vie said, the Apeks regulators are mostly the same. As for the first stages the DS4, DST, ATX50, ATX100, ATX200, and Black Pearl first stages are essentially the same in performance. The differences between these first stages are cosmetic. The hose routing on some is fixed while on others there is a swivel turret. There are external finish differences, port placement, number of ports, and size of one of the low pressure port differences, but the internal workings are the same.
The same holds true for the various second stages. The ATX series second stages have different cosmetic front covers but the same internal parts, so long as you are comparing apples to apples. By this I mean if you are looking at two second stages and they are the same size and both have cracking pressure adjustment dials on them, then they are the same. Apeks does make TX series second stages that are a little larger than the ATX series second stages, of course the physical size will make these a little different, but they are essentially the same. Apeks also makes some second stages in the ATX and TX series without the cracking pressure adjustment, these will of course have this difference separating them from the second stages with the knob.
So, a less expensive DS4 first stage paired with ATX50 second stages will perform the same as the more expensive ATX200 set. The differences are the ATX200 has two high pressure ports, a single 1/2 inch low pressure port and three 3/8 inch low pressure ports, and the basic shape with three ports coming off of one side and the other 3 from the other. The DS4 has only one high pressure port, all low pressure ports are 3/8 inch and the ports come off of the fundamentally circular first stage at fixed positions.
If you want to spend the money on the ATX200 there are valid reasons to do so, but fundamental performance differences are not among them, this regulator will not deliver more air faster or at a greater depth than the lower priced DS4.
Mark Vlahos