AD7705 vs TM7705?

I did exactly the same as you. While looking for an AD7705 evaluation board, I came across the red TM7705 based board on AliExpress.

Here is the "Titan Micro" data sheet. And that of AD7705. Don't read Chinese, so I haven't tried to compare the two chips in detail. I have only found that TM7705 is pin to pin compatible with the Analog Device chip. Convinced that it also has similar characteristics.

Does anybody of you know the differences of the AD7705 and the TM7705?

Yes, whoever produces the TM7705 knows the differences.

It's what it's called a clone: a device meant to fulfill the same role, with the same specs. However, the fact that it's much cheaper probably means the manufacturer of the TM chip needed to go for a few compromises: Higher power consumption, lower accurracy, higher probability of error, more noise, higher thermal dependency, less supply noise rejection, lower safety margins for ESD and overvoltages, that kind of thing.

That needn't be a bad thing: There's plenty of clones that are as good as the original for many use cases. If you go back in computer history, you'll find that a lot of progress was made due to people first cloning a successful device, then improving it (i386, the 8051, opamps…).

However, the point is that a clone is not the same device. It's simply not. So, a useful clone would come with a datasheet that you can read on your own to verify it does what you need it to do. Just like the AD7705!

Now, you can't find a datasheet for that clone. So, that's a bad device: All you really know about it is its name, and that maybe the manufacurer of it wants it to be used in places where the AD7705 might be used. Under which conditions? We don't know. With what guarantees? None.

So, it's the same for clones as for any other electronic component: No datasheet, no sale. Buying something that you don't know what it'll do in the future is a terrible idea, no matter how cheap it is.

I assume you went through quite a few ADC datasheets before you selected the AD7705 (seeing it's a speciality device); you know that you have special requirements. If there's no document from the manufacturer that states their device fulfills your requirements, you can't buy it.