If your face is round, I suggest you to wear butterfly sunglasses. And moreover, you should choose rectangular not circular sunglasses. Because this kind of sunglasses will make your face look smaller and make you look lovely. If your face is rectangular, you can choose butterfly sunglasses as well. Wearing butterfly sunglasses can make the contour of your face clearer. However, when you choose which kind of sunglasses to wear, you should choose the right suit to wear and consider that whether the color of your sunglasses and your cloth match perfectly or not.
Many people can see good with butterfly sunglasses. You should not rigid obey the theory on choosing sunglasses according to your face shape. It is also depend on your personality. Just try on and test it out by yourself. You shall form your own appreciation to fashion. Personally, butterfly sunglasses are not suit for people who have normal butterfly sunglasses.
It's hard to say which kinds of face shape are suitable, for there are many types of butterfly sunglasses. And also the color of the sunglasses is also important. People should choose the right frame which can match your face shape. Generally speaking, if you have the round face, you may have to wear the round frames. And if you have long face, maybe you should choose the long frames which can match your face well. Besides, you should know that they were one of the brands which are to actually design eyewear that is protecting the eyes from the sun while still looking good. They have perfect the materials used in the frames. Anyway, just go and choose them to have a try. I think you can pick one that fit you.