Should i do yoga or cardio exercises?

im 15 and weigh 123.5 and im 5'5" my goal and the weight i plan to be is 110. i know this sounds stupid but i want a body like Miley Cyrus. I eat very well so all i need to know is what kind of exercises i should do. i currently do 3 days a week of spinning classes or some form of cardio. what i'd like to know is should i do yoga or pilates too or instead.
i dont want to be very muscular but i want to have a flat stomach and thin legs. im not trying to have to be strong or anthing i just want to be tiny. i dont know if that makes sence but yeah.

first i'll say DON'T USE ANY WEIGHT LOSS SUPPLEMENTS. i only say that b/c of your age...and b/c a lot of them are either rip offs or unhealthy. i'm not going to type up a whole fancy paragraph for you but here's what i think you should do...it worked really well for me.
i ate 4-6 SMALL meals a day and drank a lot of water and green tea, while cutting out junk food and sodas.
i did BOTH cardio exercises and yoga... they're both really awesome ways to burn off calories and tone up. :)
i worked out 5-6 days a week for an hour.
friends were already seeing MAJOR changes in a couple of weeks. sounds like a long time, but it's not.
there's also a REALLY awesome program i'm trying out now: P90X. you've probably heard of it. it's a little expensive but it works WONDERS!
don't obsess over your weight- based on what you've said- you're already a really healthy girl but you're looking to celebs for your image...which isn't always the best way to go.
best of luck!