Let's start by saying Swivels & Angle Adapters are not the same thing.
Angle Adapters have 2 points of swiveling; at the hose & at the reg.
Swivels (like Omni Swivel) have 3 points of swiveling; at the hose, at the reg, and in the middle of the swivel ball itself.
Go with a 90 Angle Adapter, any brand (or un-named) is fine.
A 70 angle adapter would work too. Those are usually used for FFM's w/ comms, where a push-to-talk button would block the 90 angle adapter.
In my experience, I don't feel any difference between 90 & 70.
Swivels are best suited for over the shoulder with very short or long hoses. The added middle swivel point in swivels allows a huge tension relief.
For under the arm, that middle swivel is essentially not utilized at all with any normal head movements.
In terms of service & maintenance, Angle Adapters take a single -010 oring as service. And it is replaced as needed, same way your orings on standard LP hoses are replaced.
Swivels require a service interval like a regulator. OmniSwivel's tech manual mandates every 300 hours or annually. The cheap non-branded ones you'll find have a notoriety on scubaboard for breaking down and leaking before the 1st year of use is over. I also have never found part kits for the non-branded one.
Part kits are important for swivels, because for OmniSwivel's kit you're replacing 5 orings, a teflon washer, a metal nut, & a metal screw. They're not prone to failure if maintained properly. They did have bad failure incidents during their first few pioneering years, but it's corrected now with maintenance; mainly changing that metal screw & nut annually.
