TLDR; I kinda screwed up. Yes, it is always true that AC points are less interesting if you're being hit with advantage.
I have posted an earlier answer that I want to correct. While the maths in it are correct, they are not actually taking into account what is most relevant for the game mechanics (I want to remind that I am not a dnd player). I will in this answer correct this with mostly the same maths, but a slightly different point of view on them which will alter the actual practical conclusion. Thanks to Matthieu M., Neil Slater, and Dulkan in the comments of the earlier answer for pointing out the part that I was missing about what actually matters in game
Odds of getting hit with and without advantage:
We will here use odds instead of probabilities to discuss chances of getting hit. That is to say, we will discuss "you have one chance in x to get hit". You could also think of it as "it will take your opponent x attempts to hit you". This x will be much more relevant to your game experience, and will show why AC is indeed less valuable if hit with advantage. From here on, I will refer to it as the required attempts to hit.
Here are the required attempts to hit a given AC with a +4 attack modifier, with and without adavantage:
See how the curves rise more and more sharply? That illustrates what commenters had been pointing out: the higher your AC, the more benefits you get from raising it.
We also see of course that it take significantly less attemtpts to get hit with advantage, especially for high ACs. This is what changes the conclusion.
What benefits do I get from raising my AC:
We now discuss how much gains you can expect from raising your AC by one more point. Of course, as we established above, that depends on your current AC: the higher it already is, the more benefits you will get out of one more point.
Here is a graph showing how many more attempts it will require for your enemies to hit you if you raise your AC by a single point, depending on your current AC:
Let me walk you through it: see for example that the value at 21 AC is almost 2 without advantage? That means that going from 21 to 22 AC will require your opponent to take 2 more attempts before they can actually hit you!
We notice two things: in both cases, as already discussed, you get much more benefits from investing in AC for higher AC values. And you can also simply see that the gains you get from investing in AC are always higher without advantage. So the value you get out of AC points is always reduced if you are being hit with advantage
N.B: In also checked the math in terms of relative gains but did not include it here so as to keep it rather short. It does not alter the conclusion. AC remains less attractive if hit with advantage
The math details:
The required attempts of being hit is simply the inverse of the probability of getting the hit. So without advantage, it is given by:
with advantage (rolling the dice twice and keeping highest result, if i understood correctly):
The gains can be obtained from derivation of those odds with respect to AC. The derivatives were calculated numerically by finite differences (this is more accurate than theoretically deriving it, since the AC values are discrete and can only increase by increments of 1)