I think it’s a matter of preference. I had no choice because one side the wall was too bowed it needed a cover with side splash. I like without.

My preference is a 6” splash all sides - I think it gives a finished look & is another nice design element. I agree the 3 cm quartz is too ”blocky” as you aptly described it. Even if you decide to do back only & your side edge wall joins don’t need a cover, have you thought about some kind of small scale tile? it would look nice going very close to bottom of mirror edge & behind the faucet is where all the splashing will happen.

I have a frame built for the mirror that will close the gap that you see, and can lower the mirror a bit more if needed, although i like there being a clear separation between mirror and countertop

My vote would be WITH sides. It looks more complete.

When wee remodeled amaster bath, I came home to a countertop with side splashes and it reminded me of a dry sink. The sides came off.

No sides. I agree with you that it looks cleaner without and it is far enough from the sink that it is not needed to protect the walls

I could go either way, but since you already have them, might as well use them.

No

If only a backsplash I think looks nicer at 6” to do it’s job effectively

Looks similar to your size.

I hate cleaning the dust off the top of the backsplash.

I think adding the sides makes it look like a public bathroom.

My vote is for no. I didn’t do side splashes in any of my remodeled bathrooms. I do prefer it to be a little larger/higher like jackowskib’s pic above. Are you changing the builder mirror? That would make a huge difference as well.

IMO what you have is useless as a backsplash it serves no purpose you need it to be bigger and I do not like side splashes except on tiny kids bathroom vanities where water goes everywhere.

The sink looks fairly far from the sides. The purpose of a backsplash is to avoid splashing the walls. It seems unlikely you will splash the sides of the walls from the sink in the middle. You can see in our image below we didn't use a sidesplash, only a backsplash.

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Neither wall is anywhere near the sink therefore that eliminates the need for the side splashes which then gives the space a cleaner look

I chose my bathroom vanities with 4” backsplashes.

Definitely no side splashes in my bathrooms