Jon, the Apeks second stages 50, 100, 200 are basically the same. The 40 does not have an adjustment knob (can be upgraded though) and the 20 is a 40 with a plastic spacer replacing a heat sink. They should all breath very similarly.
The first stages show a bit more diversity. I do believe there are slight differences in flow between them but not enough to make a difference. Also the unsealed regs can be upgraded to sealed but I am not sure of the cost. The 200 has a replaceable hard seat which is good if the seat gets damaged. If the other first stages get hard seat damage then they are paperweights.
Thanks! I will probably look for the 50 or 100. If I can get a good used 200, then it will be all good. But it is good to know that there really isn't much difference between the 50, 100 and 200.