If you put a 128x texture on a 512x item, it's essentially still textured 128x, maybe even a 32x if you decide to stretch your texture to 512x
If you put a 512x texture on a 128x item, it will resize the 512x to 128x and then stretch it to fit your textured object, turning out as 128x quality
Pick either way, they are both going to be 128x quality.