I want to buy a laptop and with the desired product there's only the option of a 64GB SSD or a 128GB emmc hard drive. Unfortunately, I can't find anything that would help me on the subject.
SSD is better.
If it is too small, you can easily replace it with a larger one.
What is not possible with EMMC. These are soldered-on chips.
Both are far too little. 256GB SSD should be at least. You can forget about EMMC.
I would advise you to go for the 128 Gb variant, as 64 Gb fill up very quickly, especially with a laptop.
Maybe even more would be recommended, could you give us a little more information about the model, the price and the usage?
For Windows:
With 64GB you will traumatically have a laptop that barely manages the typewriter. From 128GB you can talk about Windows.
And SSD better than EMMC
It will probably be both eMMC storage, because the NAND storage is also solid state. Better get another laptop.