Several years ago, I bought a Legend reg. I would not hesitate to recommend it to others. The ACD has been mentioned a couple times. This is a feature which hopefully you will never need, but if you have a momentary lapse of concentration, you will be glad it is there.
Speaking to my Legend, it is more than 10 years old and it still breathes effortlessly. IMHO, I think that you would likely be very happy with a Leg3nd and that it would serve you well for several years.
I could be wrong, but I think that the two reg brands that you mentioned, AquaLung and Apeks are to a degree affiliated. I believe that Apeks is a "sister company" to Aqualung. If that is true, then their regs should, in theory, be fairly similar in terms of performance and reliability.
As for the Oceanic Delta 5, I have never used one, so I can't speak to it, but I do believe that within a given price point/price range, regs from different manufacturers will tend to offer similar performance with slight variations on the features.