While your shield may be bigger, you still take more damage at base, so it doesn't affect you except in the niche circumstances where attacks do a fixed amount of damage. I even done the math to double check for myself (20% damage reduction compared to +25% extra HP and healing, you need +25% healing if you want them both to feel the same) and irregardless if whether it was a shield based on the amount healed or a shield based on max HP (I actually made a spreadsheet to test this and it is the case).

So really, the only benefit is the flavour of the stance. They are both the same mechanically speaking, so you can essentially call them the same stance. There is no simplicity there, just the same thing in a different coat of paint.

Going away from tanks for a second and talk about healers. I will admit, I don't know what a healers mindeset is in high end raids, so I am speculating. I know healers know roughly how much their heals heal for. If they see tanks with differring HP amounts, would they then have to learn 2 different values for their heals and swap between them when the tanks happen to swap? I don't know how much of an issue this is, if it is even an issue at all. If it is an issue, then really you want consistency across the board.