In discussions on BPW harnesses, there's a lot of opinions floating around that a harness isn't "tec" if it has a sternum strap, unnecessary D rings, or comfortable padding, etc. It felt like it was really just a matter of in-group/out-group mentality, with the bare bones harness having been around before comfort harnesses, and since that's what all the cool kids are using comfort harnesses became heresy and a mark of a "tech-wannabe."
I'm interested in a comparison of the actual functionality, though. What difference is there in stability with the shoulder strap + sternum strap setup, versus the shoulder + crotch strap setup? What about harnesses that have both a crotch strap and a sternum strap?
I've used both types (Diverite Transpac, now Aluminum BP with self made 1 piece Hog harness)
Sternum strap - Not necessary. But you'll probably feel more secure with it. Use it if you like it, but functionally it's not needed on a properly adjusted harness.
Unnecessary rings - Up to you I guess. I don't find them necessary though, but I don't like to hang unnecessary things off my harness.
Comfortable padding - Good to have especially below shoulder straps. You might be fine with bare webbing in temperate conditions, but when you're carrying 30lbs of weights for drysuit diving in cold waters, and having to hike a distance to the dive site, you're going to wish you had padding.
Crotch strap - must have. This is what cinches the whole rig tight on you (along with your waist strap).
You didn't ask about quick release buckles, but here's my take. When you're carrying multiple tanks and stages in a cave/wreck, you probably don't want a potential failure point from quick release buckles. But if you're diving recreationally with one tank, quick release buckles might be a good thing.
Just take note where they're placed. If you are going to clip off lights on your shoulder D-rings, you probably don't want them near where you're going to strap them with rubber bands (like on the Transpac).
Take note though, some harnesses I've seen, like the Transplate comfort harness, is really hard to adjust if you're not just using the quick pull straps. You can't really move the position of the shoulder D-rings around easily because of how it's configured.
---------- Post added December 31st, 2013 at 04:19 PM ----------
Not wanting unnecessary ornamentation makes sense if cost is a concern, but what about extra D-rings is a liability? From the way some people talked about them in other threads it felt like drowning from D-ring entanglements was a daily occurrence or something crazy. Most scuba gear appears full of designs, etc. that don't serve any practical purpose and are just there for aesthetic reasons, so having extra D-rings because it looks cooler makes sense if there's no real downside.
Clipping off/on things on shoulder D-rings will be done by feel when you're in water. If you have additional D-rings, it'll be confusing to figure out which one to clip on to, or where a torch was clipped off. By having only 1, you remove this ambiguity.
Sternum strap interfering with some people's drysuit inflators makes sense. But in designs with no crotch strap is the sternum strap not necessary? I hate the sternum strap on my jacket BC, but am not a big fan of crotch straps. Wondering if the waist/shoulder straps will be enough support when diving heavy doubles or if crotch strap is necessary in that case.
The sternum strap cannot replace the crotch strap. The crotch strap is what pulls the whole rig down on you, and tightens up the shoulder straps and keeps it there. Without it, your rig can float upwards, and the sternum strap cannot mitigate this.