Well for starters, both phones are great phones and for all intents and purposes the iPhone Xs is a more improved iterative replica of the X and even though they look exactly alike, there are actually quite a few key differences between them.
Chip: The iPhone X comes with the apple A11 bionic chip which is an exceptional SOC which was basically the most powerful smartphone chip as at the time it came out. It was built on a 10nm process which was leagues ahead of the competition when it came out. The X's comes with the apple A12 bionic chip which is quite a bit faster and more powerful than the A11, plus it's built on the 7nm process and as such it's more more power efficient.
Camera: Both phones have the same camera, the difference lies in the size of the sensors. The main 12mp sensor at the back of the X is 1.2 microns and that of the Xs is 1.4microns, as such the Xs can take in more light for better low light shots.
IP rating: The X is ip67 rated and as such is dust proof and water proof up to 1m of depth, while the Xs is IP68 rated and is dust and water proof up to 2m of depth
Durability: From what I've seen, the Xs is by far a much stronger phone than the X and it's to me the strongest and most durable iPhone ever. In fact its even more durable than the new Samsung phones that where just released and this is coming from someone that doesn't even like iPhones, but I just have to say the truth.
I hope this helps.
The iPhone X with the iPhone XS actually doesn't have that much difference in performance and quality, so you can say that these two smartphones have almost no difference at all. Both of these smartphones use the same Bionic A12 processor, have the same choice of internal capacity (64GB, 256GB, or 512GB). The difference is on the iPhone X which does not have a choice of 512GB internal capacity; only 64GB, 128GB and 256GB. That's where the most prominent differences in the two smartphone brands while other differences are not so visible to the iPhone X and iPhone XS users.
Whereas in the RAM section the iPhone XS and iPhone X both have 4GB of RAM. But this difference is not really necessary to think about because when you hold all the iPhone, the difference in multi-tasking performance is not so pronounced. The iPhone XS and iPhone X each use the same A12 Bionic processor, but the iPhone X loses because of the more limited choice of internal memory so that it makes the iPhone X slightly less rivals than the iPhone's iPhone XS.
Bionic A12 processor itself is claimed to have 15% faster performance than the iPhone X which only uses the A11 Bionic processor. So, technically, the iPhone X loses and if you think too much about the iPhone where the performance is better and faster, then the iPhone XS is the choice. The point is, between the iPhone X smartphone and the iPhone XS it only has a slight difference, that is, the facilities on its internal memory are larger than the iPhone XS iPhone XS. However, this difference does not make the enthusiasts of the iPhone X less at all.