From what I can see, Apeks seems to be the higher end of Aqualung ala Acura versus Honda.
While this was definitely true when Aqualung first bought Apeks, I think at this point AL(Aqualung) has stolen enough ideas from the Apeks line that the top of the line AL reg (Legend LX) is essentially the same as the top of the line Apeks TX200.
AL will never have the tech cred that Apeks built up, but I imagine that any performance differences are gone. They are using the exact same technology now.
If this is true (I feel it is), and you are buying new (it seems like the OP is), I would recommend the AL range over Apeks for a couple of reasons:
1. More shops stock more AL parts (Many AL dealers simply do not stock Apeks. They should but they don't)
2. Big parts are much cheaper for AL regs. And you won't ever need to replace a body because AL uses replaceable HP orifices. Some APeks regs still have fixed orifices. That means if the reg floods (like a used one I just bought) and it is not rinsed out, you can waste an entire first stage body because of corrosion on the first stage orifice. Even without the difference between a replaceable part and a fixed part, AL's parts are just cheaper overall. I love Apeks regs, but getting burned by buying a wasted first stage has kind of soured me on them.
3. AL regs (and Oceanic regs) now have a sealing mechanism on the first stage that would prevent the flooding of a first stage in the first place. I would absolutely not buy a new reg that did not have this technology.
AL calls it ACD (Auto CLosure Device)
Oceanic calls it DVT (Dry Valve Technology)