Both the SP MK25 and the Apeks regs (DS4 or better) are both quality regs, you can't really go wrong with either one. That being said:
Models To Consider:
BTW all Apeks regs that are environmentally sealed are basically the same first stage, with some shape differences or a replaceable HP crown on the top of the line. The second stages have two versions that are basically the same as well, with the TX models being larger with a larger exhaust tee than the ATX models, they come either with or without an external adjustment for cracking resistence. For SP, the models to consider are MK25 paired with S600 or G250 (or whatever they are calling it this year).
As for other models of these lines to not consider: don't bother with the Apeks' that are not environmentally sealed (ie T20, US4) or ones with turrets (maybe exception for stage bottles). As for SP, the MK16 (diaphram), has WAAAAY to many parts and the MK2 (piston) is very simple and reliable, but might does not match the Apeks or MK25 when it comes to performance at 100' on nitrox. In performance, I have a MK16/S550, and the Apeks and SP MK25 both breathe much better. The MK16 paired with the S600 or better might have better performance??
Features to consider between MK25 and Apeks/DS4
Routing: Some like the MK25, some like the Apeks DS4, some like the Apeks TX100 (AKA flathead 6). Think about singles, doubles, and stage bottle routing.
Piston/Diaphram:MK25 is a piston design, Apeks are environ. sealed diaphrams. This may be a concern for some people/environments. Check the stats to see if there is a volume of air delivered difference.
Cost:Obviously this can vary from place to place. If in a real bind, some people have had success getting DS4s from Europe, however please keep in mind warranty implications of this. Also the European second stages might be slightly desensitized due to CE standards.
Warranty:SP is free parts every year if you service every year. Apeks is free parts every other year, if you have it checked by a tech at least every year (available US only).
Servicing:If interested in servicing yourself one of these days... if you work in a shop, it is easier to get training to become "certificated" (they don't certify you any more, they just give you a certificate) by Seaquest to service Apeks, which BTW are similar to the Aqualung Lengend from a repair perspective. SP training is harder to get, and more expensive. SP parts can be harder to come by than Apeks parts. SP's also need more specialized tools, but these can be purchased from PeterBuilt online.
Dealership:The SP dealership is harder to get and keep in some areas than Seaquest (Apeks distributor in US). If relying on a tech for service, make sure the dealer has a good, experienced tech for that line. Keep in mind that Seaquest training is easier to get and SP training is harder, so *sometimes* new SP techs are better than new Seaquest techs. But an experience tech is wins hands down. Also, don't forget to communicate with your tech as to how you want the reg tuned!
Adjustability between overhauls: This may be a consideration for you. Both SP and Apeks probably have the same ease of adjustment. For Apeks the IP adjustment is under the Environmental seal, but it only takes one wrench to get that off. For SP, I'll get back to you.
Performance: There's the rub. Very subjective. Only way to know is to try them yourself. Remember that the most effort of breathing on modern regulators is on the *exhale,* the inhaled effort is often more a mental thing and greatly depends on how the reg is tuned/maintained. Both regs are very dry from experience. I haven't dove a MK25 much, but I do know that as the HP seat wears in a little on the Apeks the performance can dip a little; they breathe amazing out of the box. But then again, I'm only noticing this after 130+ dives. I also have the bad habit of setting up my gear the night before and leaving the regs pressurized... not the best thing for the seats. If you were really concerned, you could overhaul, or at least replace the HP and LP seats, each year.
Team:What is your team diving? If everyone is diving Apeks, it makes it much easier to swap things out, borrow parts, if something breaks in the field.
You:What other regs do you currently have, this may also play a role in your choice.
So no hard recommendations, but things to think of when making your choice.
Personally, so you know my bias, I dive Apeks DS4, ATX50s for everything: doubles, singles, argon, and eventually stages/deco. I have received training from Seaquest on servicing the Apeks line, but have not worked at a tech. (I do have a MK16/S550 left over from my pre-DIR days slotted for a shallow deco reg).
