I've always looked on the old steel "jerry-can" as being the epitome of product design excellence.
The dimensions make them easy to pack together (width = 2 x depth), and easy to manage (not too heavy). The 3 handles mean than one person can carry 1 or 2 cans at a time, or 4 if they are empty, and two people can easily share the carrying of 1 can if necessary. They are the right size to be comfortable to grip, and free of palm-irritating seams. And they are well-placed for lashing, and they offer some protection for the lid.
The body is cheaply built, but very rigid, while patterned to allow the sides to expand a bit under pressure.
The flip-up lid is a masterpiece. It seals perfectly, even under pressure, and the seal is easily replaced. It can be locked closed with a padlock or a wire/plastic seal. The lid can't get lost. The simple separator plate in the mouth makes the can glug-free when filling or emptying (unless you tip the can at extreme angles). This means that no secondary opening is necessary. The lid is held open and out of the way by its very simple but clever hinge. And the flanged mouth and lid-retaining brackets lend themselves to all manner of crafty add-on spouts and feeders etc. which can attach/detach in seconds.
Altogether a hard act to follow!
Those Specter cans look great too. Far better than the rather disappointing plastic Nato water cans we used on our expedition. Obviously, the use of plastic instead of steel is their biggest advantage over the "proper" jerry cans. Corrosion is the Achilles heel of the steel cans, especially for water storage (but petrol also doesn't inhibit rust very well). And the fact that they don't get dented and (presumably) don't suffer from chafing are big pluses,too.
Sadly, it does seem to mean the demise of the perfect lid
I imagine it's just not possible to have a plastic lid that is secured in the same way as the steel ones. And a thread instead of a flange means that the lids have to come off completely, and get dirty/lost, or dangle annoyingly by their tethers.
For water, there's no question, I would opt for the Specter cans, but I think I might, on balance, still use Jerry cans for diesel.
Btw, does anyone know if there any lids for the Specter that would facilitate the use of an electric pump for water distribution? Ideally, I would like to be able to have a bank of water cans, and simply move the pump's pick-up from can to can as needed. I never found an elegant solution to use for our Nato water cans.