I have seen both, and I like them both, but they are very different.
Yuna is a doll. She is cute, fun and friendly. She is not quite as well built as the pictures might lead you to believe, but personally, I like smaller tits. Lot's of fun in the shower. Very responsive in bed. She is in the Amber / Yunjin class for GFE. You will have a good time.
Elsa, to me, is really hot. Cute face. Almost perfect body. Amazing perfect tits. There are pages of arguments in the forums on whether or not they are real, but if fake they are amazing work. She has the best legs of any girl I've met, and I had her modelling them for me in her Jimmy Choo heels. She is more real and direct than many other girls. No fake moans. No "Eeets so beeeg" bullshit. She's more like a real 20 something girlfriend than an escort. With Elsa it seems it is much more of a YMMV than with Yuna, and you have to work harder to earn your goodies, but to me, Elsa is younger, hotter and more fun. I've already seen her three times and I'll probably see her again next.
A consideration is that Monday is Yuna's last day.
Just an update. I ended up seeing Yuna again this afternoon. Holy shit she is amazing. If you can book her before she leaves tomorrow, do yourself a favour. Some girls let their performance slack off before they leave. Not Yuna. She wants us all to miss her, and I will.