Fascinating stuff, especially about the shield grips.

I'm assuming the Roman held his scutum out in front almost like a curtain, which makes sense if he expects javelins. IIRC shields in papua-New Guinea and Northern Australia were whirled to deflect javelins: the reinforced central spines of Celtic shields depicted in EB remind me vaguely of this use. Perhaps he held it closer to the body when in melee.

The hoplite depicted seems to hold his shield out a bit angled: maybe thats a better stance if he expects an overhand spear thrust? Definitely strapping the thing to your arm means more of any blow will hit the arm, so perhaps the aspis is less of a static defense (soak up force) and more of a "wave blows aside/use it to push" device.

My guess is the Roman shield is more of a javelin deflector but is versatile enough to be a decent melee shield vs a variety of weapons. The aspis seems more like a specialist tool for push-of-pike scenarios and maybe less suitable for quick whirling to deflect incoming thrown weapons, or crushing blows from axes.

I imagine the Greek stance a little more side on, with the shield angled out in front and the spear delivered over hand from behind and over the shield wall. This might leave the belly/thighs more exposed but if the enemy is also hopites fighting overhand there's less threat there perhaps.

Maybe the Roman stance is a little more squared up? Perhaps when using the gladius, although javelin-throwing stance would be similar to the Greek stance (perhaps in a looser formation as I've seen described in other threads).

Just a guess.