Neither.
It’s only if the object is loaded at runtime / and it’s memory consumption is related to what MIP level is on screen.
Mixing monkeys.
Texture streaming is a Silverback, material insrancing is a baboon.
You can and arguably should use one master material for almost everything - if you build it correctly.
And create a separate instance for each variation needed.
Texture streaming really doesn’t apply here. You can force VT (virtual texturing, a zombie pirate when compared to a monkey?), If you know how it works, and how to take advantage of it.
Different meshes with different materials doesn’t really “fit” well with the concept.
But you could use the concept to unify 4 4k textures into a single 8k one.
You then Section off the UV in the model to use only the relevant quadrant, and force VT - you would see some performance improvement there when an item loading 1 quarter of the texture is not on screen.
Ergo, it’s basically the same as loading separate 4k textures (there’s more to it, but that’s the gist).
Consider it yet another “vanity plate” that the engine has failed to properly implement over the past 2 years.
Without a single doubt, texture reduction.
Next up is proper instancing.
Using an HISMC for the road pieces may reduce the texture cost a bit.
Same for any other loose object. Placing it as foliage will automatically instance it, possibly providing some savings when the object is used a lot.
Things like grass, definitely don’t need 4k textures when they are barely 2px large on screen, for instance.
The engine has a debug view to help you reduce sizes as needed. You should look into some of the Optimization courses for lighting.