I use them in different situations. I use "getting ready" when I'm like putting make up on, getting dressed or taking a shower, doesn't matter when, morning, evening or for a party. I guess I use "getting ready" before you leave the house or someplace else. I feel like "getting ready" is the short form of "getting ready to go (+ add place)". So you're not there, at that place, yet but you're doing what you need to do in order to be ready to go. As for "preparation" I would use it for school situations, like I'm preparing for an exam or test. Or when you're cooking: like I'm "preparing dinner" I don't know if you understand, but if you let me know I can explain better :)