The plain steel block has 500 hitpoints and a mass of 1 ton, the hardened steel has 1000 hitpoints at a mass of 2 tons, and the combat steel block has 2000 hitpoints at a mass of 4 tons. The number of hitpoints per ton is linear, and no block type offers any advantage over another in this respect. Four blocks of steel is the same as one block of combat steel.
The cost comparison reveals a significant difference though.
80 blocks of steel costs 400 iron ingots to produce, while 40 blocks of hardened steel (which gives the same number of hitpoints) costs only 220 iron ingots, plus 20 of cobalt and 80 sathium. Of course you could produce 70 to 80 blocks of hardened steel for a similar resource cost to the steel blocks, but gain a significant amount of hitpoints. The combat steel is the cheapest by far in terms of hitpoints per resource. 20 blocks of combat steel offers the same hitpoints as 80 steel blocks for the low price of 116 iron ingots, 16 cobalt, and 59 sathium.

The conclusion: while no block offers an advantage on weight, combat steel offers a significant resource reduction for a given hitpoint configuration; however, 3 blocks of combat steel will be more expensive (slightly) than 3 blocks of plain steel. This is offset (in theory) by the idea that a single block of combat steel can do the same job as 3 blocks of plain steel.

I began an experiment with several test plates.
Plate A: plain steel, four blocks thick
Plate B: hardened steel, two blocks thick
Plate C: combat steel, one block thick
Plate D: plain steel, four blocks thick with each separated by one block of air
Plate E: plain steel, four blocks thick with each separated by two blocks of air

Each plate was broken into two halves. The top half was subjected to rockets and the bottom was subjected to pusle lasers.

Rockets: (4x manual rocket launcher)
Plate A was fully penetrated by the second shot.
Plate B was fully penetrated by the first shot.
Plate C was fully penetrated by the first shot.
Plate D was fully penetrated by the third shot.
Plate E was fully penerated by the fourth shot. (each layer took a full shot)

Pulse Laser: (6x manuual pulse laser)
Plate A: 14 shots
Plate B: 13
Plate C: 14
Plate D: 14
Plate E: 14

The results show that air-spaced armor layers are vastly superior against standard rocket salvos, but offered no additional protection against non-aoe damage types. This suggests that layering can increase the amount of time a ship can survive a battle. Nearly no damage passed to unexposed layers with a two-block gap present.

I rebuilt test plate E to it's earlier configuration and re-configured my launchers. One launcher was placed in front, the next was set three blocks back, the next was set three more blocks back, and the fourth was set another three blocks back in an alternating manner:

back --------------------- front

OOO[ >
OOOOOOOOO[ >
OOOOOO[ >
[ >

O = block
[ > = launcher

This way, each rocket is staggered. Even though they launch simultaneously, they hit one at a time. This time difference between each rocket is enough for destroyed blocks to be removed.

I fired another salvo at the test plate. Four blocks thick with two blocks of space between each layer. A small hole appeared, only one block was missing, but I went to investigate before firing a second salvo. There was a block missing from each layer. Complete penetration was achieved on the first salvo. I noticed another effect of this placement as well. The damage was less spread out than before. My initial setup was composed of 4 launchers in a 2x2 square formation, and it caused a much wider damage pattern. Enough of a hole for a small sv to fit through. The new method created much smaller holes, but the depth was much greater. This would likely result in greater damage to internal components like ammo boxes and fuel tanks. The staggered method completely negates any advantage of spacing armor layers apart.

I repeated the test with a combat steel version of plate E, 4 thick with two-block gaps, with both launcher configurations. The 2x2 configuration didn't make holes any larger than the staggered configuration. In general for greater penetration you should target the same area for multiple salvos, and for a wider hole you should spread them around a bit by targeting different points.