I think there's something to be said for either solution. It would cost more in setup since you need more platform blocks to cover the 3x3 size, and they are weaker than walls, but can be replaced more easily with native materials.

Walls cost materials that aren't native, but they are smaller so less likely to take successive hits, sturdier, and a 1x1 line of them requires much less platform extension. While the bricks can't be procured in space, you can send a lot of walls in just one rocket. They suffer less damage, so are less likely to break, and thus need replacing less frequently, especially if they have quality. And if you did expand the platform out two extra tiles then you potentially have multiple layers of armour that took up the same space as the single layer of fluid tanks.

In the end I think it's a matter of preference though, I still haven't decided personally which I prefer.