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Seriously, i do like H's because they are modular, and can be conversed to a manifold, reversed, even blind plugged, and can be more versatile on first stages/hoses routing
Some may argue that it has one more o-ring adding a failure point, but is the same when you discuss solid bar versus isolation manifolds.
All that said, i would go H, and that's what i use on my steel 18 litres
Y valves have one less failure point than H valves if that is something you are concerned about.
H valves are modular and the 2nd first stage attachment point can be rotated for convenience of you want your first stages to sit. I beleive the 2nd attachment point of an H valve might can be removed and then joined to a minifold for a doubles setup.
While living in Europe, I primairly used an H valve. In summer I used 1st stage, and in winter 2 1st stages, I had a 3L pony bottle slung under my left arm regardless of season.
While a doubles may be better, I have upper back issues and the weight of doubles causes me a lot of discomfort when I am not in the water.
If I was to choose between H and Y, I would choose H.
The Y-valve we typically see here in the USA is the very simple, old-school U.S. Divers or Sherwood type that accepts A-clamp/yoke regulators and has a single dip tube. I often use these for my solo dives here in MO/AR when I am doing single tank dives to moderate depths, three-season diving.
I prefer Y-valves over H-valves for their simpler construction, because they allow a simpler routing of my hoses, and because it's much easier for me to reach both valve knobs. Downsides (for me): not DIN, cannot be used on HP (3,500 psig) cylinders as is, and a bit of a pain to carry the cylinder by the valve.
All my steels are M25, as they are from europe.
About some decade ago there was a well talked and discussed accident here in Brazil, where a late tech instructor (no, she didn't died on the acident) have a valve bursting into the sky after "machining" an american valve to fit an european cilinder.
Fortunatelly no major injuries from that