The XTX200 is the high end, although in terms of actual performance, I have trouble distinguishing it from the XTX50. I have a couple of ATX200 second stages and really like them. I suspect part of that is subjective perception, not objective performance differences, however.i know the xtx200 is considered by most apeks users as the cream of the crop. and i do love mine. but is there really any noticeable difference when using them or maintaining them etc. is the dst a big step down for any reason
No, the DST is NOT a big (or even a small) step down. I am probably a bit biased, as I tend to favor Apeks regs in general. I have several DSTs, a couple of FSTs and a couple of FSRs. Honestly, I have trouble distinguishing between them, u/w. I have used each type below 200' (with an ATX200) and find the ease of breathing to be similar. There may be others on SB who have different experiences, who will hopefully share those.
For what it is worth, I also have a couple of US4s paired with AT20 second stages, and they WILL free flow in cold (45 degree) water. So, the environmental seal is, for me at least, worth having.