Terrace covering 40m² - Concrete vs. Tile?

Yes, I really mean tiles - ceramic tiles for outdoor use. They are actually normal porcelain stoneware tiles, nothing else.

Oh, so it is slabs after all (ceramic slabs, terrace elements…). We have ceramic slabs 700x400x22 in a gravel bed.

Oh, so it's plates after all (ceramic plates, terrace elements...). We have ceramic plates 700x400x22 in a gravel bed.

Well, that might already be some help for me. How satisfied are you with it? Are the 2cm "sufficient"? So is the weight of the tiles sufficient for installation in a gravel bed? Where did you buy the tiles? Specialty store? What did you pay "back then"?

Is there usually something significantly under 60€ for 2cm high tiles? Is there anything to watch out for when buying online, or can you just buy without worries?

In every hardware store in every color from 25€ (that is then porcelain stoneware). Doesn't matter whether Obi, Hornbach or whatever they are called.
Put another way: I can't find 2cm terrace slabs online, e.g. at Obi, that are more expensive than 49.99 €/sqm (these are then the large sizes of 60x120 / 30x120 / 45x90 etc.).

In every hardware store in every color from €25 (then it is porcelain stoneware). Doesn't matter if it's Obi, Hornbach, or whatever they're called.
Put another way: I can't find any 2cm terrace slabs online, e.g. at Obi, that cost more than €49.99 / sqm (these are then the large sizes of 60x120 / 30x120 / 45x90 etc.)

I then just wonder: why should I buy in a specialty store for, say, €60-70?

In the end, 2cm porcelain stoneware is just 2cm porcelain stoneware, right? Sure, different manufacturer etc... But I can't really buy rubbish in the hardware store either, can I?

But I can't really buy rubbish at the hardware store either, can I?

No. At worst, you just have a more limited selection.

I then wonder: Why should I buy from a specialty store for, say, 60-70€?

If you don’t see any added value for yourself, then you don’t need to buy them from a specialty store :)
It’s like with everything. Why pay 30 EUR for laminate XY when you can get laminate XYZ starting at 7 EUR at the hardware store ;)

In the end, 2cm porcelain stoneware is just 2cm porcelain stoneware, right?

Not quite.
Possible differences: rectified or not? Glazed/treated surface or not? Surface texture itself.
The "pattern variety" of the décor? It can be that the tiles are "stamped" with only 2-3 pattern designs and after installation it looks awkward because the pattern repeats constantly on the third tile. If there are 4-5 or more designs and the tiles are mixed from the packages anyway, it looks more natural and coherent.

Ultimately, the quality in terms of dimensional accuracy, etc., is crucial. Some tiles "sag" or have a "bulge" which you don’t notice at first glance. Depending on the installation method and the chosen pattern, you may then have noticeable overlaps/differences at the transitions...

Sure, different manufacturer etc... But I can’t buy truly rubbish at the hardware store either?

We just had our similarly sized terrace covered with porcelain stoneware a few days ago. Considering that I originally absolutely wanted to go with wood and we ultimately switched, we’re quite satisfied.
2cm and 60x90 tiles, glued and grouted. I found a supplier online who could still give me a great price of 38 EUR, and I bought up the remaining stock. The landscaper was very surprised by the price and quality. He had recommended his building materials dealer beforehand, where the cheapest private label 60x60 tile was 51 EUR (compared to 39 EUR in November).
Our tile was cheaper, significantly better, and would have cost much more at a "typical" specialty store for this size.

Alternatively, we could have just bought from the hardware store as usual. In our case, Hornbach had the best selection in comparison (also online ;)).
With 35 EUR on average, you’re set for 60x60 and you probably won’t notice any/little difference later compared to a "comparable" one from the specialty store for 60 EUR. That at least is my experience.

Regarding thickness 2 or 3 cm, you can definitely use 2 cm if glued. If you want to lay it yourself and/or simply in a gravel bed, then at least 3 cm is recommended.