Under bino-harness is the way to go! When you're out there, ya keep the bino-harness on the whole time anyway. Might as well use it to hang your pistol off of it!
When ya got another holster that has it's own shoulder straps as well, Ugh... it's too much, it just freaking sucks and wears on ya after awhile. Especially in the heat. SoCal on this side, so I dunno, might be a non-issue in colder climates?
I like the idea and setup of the kydex holsters under the bino harness, but, the one I'd first picked up it came with an additional retention strap to go around your body, to further ensure a clean pull from the tension of the kydex. But two things... fussing with that extra strap.. and ya know, the more stuff ya have criss-crossing making contact with your skin/body, is more chance for irritation too. AND.. this other unit... it had these snaps that where so difficult to snap shut, (most likely on-purpose??) that you needed to gently using like a set of channel-lock pliers and cover them with some bicycle inner tube rubber before you carefully squeeze-em down so as to not mar/scratch the surface of the snap closure? Adn to remove them REQIRED a big flathead screwdriver or a small chisel, to pry up under the snap to pry it open? It was the kinda thing where you can tell they never thought anyone would want to put it on AND also take it off again. So thew newer style MArsupial is putting out now, just made sense to me.
Thing of it is... to practice glassing?... since I live sorta by the coast, in a city location, I also like to use my bino-harness at some nearby wildlife viewing places here local on the coast, where I've been keeping in practice by glassing up the Yotes and Bunnies and Birds and Fish living in there. Having that holster, even though empty, can just... bring ya the wrong kind of attention and looks and judgemental BS nobody should have to deal with. Let's just say... It freaks out "the normies" to see kydex holsters.
So, after I'd bought a second bino-harness to outfit my son last year. for his setup, we tried instead the newer Marsupial "enclosed" one. As my prior one of theirs would always end up having some amount of leaf matter finds its way to the bottom of the bino compartment, and enclosed style also helps keep dust off the lenses for longer. That's helpful out here in the SouthWest some times, as you can imagine.
Though slightly less rapid than the kydex, since ya gotta clear that velcro tab first... it's a total non-issue, it's very quick to draw your weapon from it. Easy-peasy. And this newer version of the simpler under-harness holster that Marsupial is now putting out? It fits my G29 10mm, and everything else lesser (9mm, .357 snubby) that I might also elect to carry, weighs less, doesn't have the annoying strap, and is fairly quick and easy to remove the pistol harness if ya want. And, it doesn't even look all that intimidating to "the normies" if ya leave it on, (empty) either.
Liked it soo much, Daddy replaced his older one with the newer style. Very happy with the minor design tweaks.
Only minor thing to note, is when placing your binos back in to the bino-harness, for the "enclosed style" be careful, for maximum stealth, to make sure to pull the bino-rentenion straps carefully out from underneath the neodymium magnet'd enclosure lid, so next time you grab for your binos you won't inadvertantly cause those retention straps to pull THROUGH the magnet-closed lid and potentially causing an unintended bit of snap-closed noise to happen when you didn't intend it too. Very minor training issue.