IMHO the difference that counts is that SP25 first stage is a flow-through piston, and Apeks is an environmentally sealed diaphragm. Thus, for me Apeks would be the obvious choice if I planned diving cold or contaminated water. Black Saphire is a great reg, but it is an item of status rather than of a better performance - like limited edition cars or suchlike. XTX50/DS4 is the workhorse in the Apeks family. With a DST 1st stage it has LP ports on a rotating turret which helps to route the hoses optimally, even though some would say that it adds more failure points. An optional 5th port is available, which is beneficial for sidemount configuration. XTX 100 has the slimmest, most compact 1st stage (FST). XTX200 first stage (FSR) is glossier and a tad bulkier, and has a replaceable valve seat which is not a biggie. The 2nd stages from XTX50 and up are all the same, just different decals. TeK 3 are ideal for backmount twins but not very handy for singles or sidemount because of the hose routing.
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