Arborist and professional mountaineers use 3 action carabiners because ANSI require it. Most of them also use steel carabiners. They put their equipment through very serious use almost to the point of abuse. As a recreational saddle hunter, 3 action standards cannot be enforced. And you nailed it about shape. The shape can be influenced by activity, as well as what you use. Oval and pear fit easier into ascenders but D shaped are stronger. So if you’re using a rope grab, an ascender, or a prusik can determine your preferred shape as well.You'll get a range of opinions on this. Just my two cents here, FWIW. I'm no expert but I found these tidbits interesting when considering carabiners:
-I was told an arborist safety standard is for a carabiner to have a three-step opening (i.e. pull the gate sleeve down, twist, and push open). Most biners I've seen used by saddle hunters are only two step (i.e. screw down and open...plus other two step styles). Depends what you decide is safe?
-Biner shape can be influenced by the activity you're performing with the carabiner and any equipment it's clipped onto.
ANSI says no for professional use. Screw lock and 2 action gates are permitted per UIAA for recreational climbing. Which hunting is recreational, so I am assuming it will be the same thing when standards come outAll my climbing carabiners are aluminum, all my steel carabiners are for rigging. It helps keep them from getting mixed up. Also anything with a screw gate isn’t for climbing. I’m sure they’re fine for most climbing applications, but they’re slower and only slightly cheaper usually, and ANSI says no.
Triple action/ double locking carabiners are very fast and very secure. Almost impossible to open accidentally, assuming the lock engages and it’s not filthy or damaged.