The Scubapro 109 is a pretty regulator but it is choked down with a small exhaust valve. Exhaust effort is just as important as inhalation effort in the breathing cycle. The CE approved BA 156 has the larger 30mm valve compared to the 26mm in the 109. You think the 4mm is not much, it is a big difference. But the 109s are pretty.
Excluding perhaps some of the D series and the AL Legend is no slouch nor the Apeks, a G250 will hang with about anything. It has a tough 30% glass filled resin body, the large 30mm exhaust valve and the VIVA control the 109 and 156 lack. Scubapro claims it is the most copied regulator, if it is not, it should be. Is the G260 equal to the G250, yes since it really is a G250 with Micro-Adjust but for some reason the G250 exhaust effort is measurably slightly less. The difference is minimal, insignificant IMO. The adjustment knob on the G250 and the 109/156 is prone to getting bent, the G260 is better in that regard, less vulnerable to damage. Regulators, particularly second stages are vulnerable to being damaged, drug on the deck, tanks set down on them, all of that fun stuff. I try to clip them off but you know, I forget or get involved in some sort of scuba mayhem and you know, excrement happens. I hate that.
The Mk V has essentially one moving part, the piston with integral knife edge, and a lower parts count than later SP piston firsts. I think it can be placed on the same level of reliability as the unbalanced Mk II. And compared to the Mk X, the ambient holes are large enough to actually rinse the first stage ambient chamber out. The "Conshelf" type diaphragm first stage is a solid unit but not on par with the Mk V for reliability. It is just chocked full of little bits and pieces whereas the Mk V is simply chocked full of goodness.
