I disagree. First of all, space marines rely on the armor/piercing threshold
@Znail mentioned.
Secondly, unless your division is tiny, you won't get 30% hardness from any single armor battalion. Hardness is averaged. In a 10 battalion infantry division, you'll get 10% hardness
at best from one added armor battalion.
And lastly, in contrast to armor and breakthrough, there is no specific inflection point to hit with hardness. All but the most dedicated tank destroyer divisions have more soft than hard attack. So high hardness when viewed in isolation is always great. But high hardness battalions usually have low org and HP. The division will have ~10% more hardness but also ~10% less HP compared to an infantry battalion in the same slot.
What is worth mentioning though about hardness is its interaction with breakthrough. Breakthrough should ideally match enemy attacks. Armor battalions raise breakthrough directly and indirectly (through hardness) lower enemy attacks (which are a proportional mix of hard and soft attacks). This is what makes dedicated armor divisions so durable, along with their armor buff. With just one armor battalion this synergy is weak though.
Unless you have heaps of army XP, which you probably won't when just getting started with tanks, you should also consider the 25 XP cost of adding an armored battalion to an infantry division. In my opinion they are better spent rounding out the proper armor template or on an initial upgrade to the tanks.