I think tortoise shell frames are classic. Some skin tones actually look a lot better with tortoise shell as opposed to the contrast with black as sometimes it can look harsh. So in that regard, it definitely depends on your skin tone and which color looks more flattering.
I think the sunglasses you're talking about are the Chanel 5076 H Mother of Pearl sunglasses. I don't own them, but i think they are really nice sunglasses and i wish i had bought them when they came out but i wasn't really into sunglasses back then. If you love them, then i suggest getting them at an authentic sunglass store as soon as possible as the internet is full of fakes of that particular model.
A lot of people are still wearing this particular style, because it still looks good. The shape is a classic rectangular frame and the logo is prominent so you can tell they're Chanel without it actually being spelled out. But the shape, size and style is the most important factor for me. Once i find the perfect shape then i look at the logo and decide whether it is too much or not enough for my taste.
Try on as many sunglasses as possible and go with what you really love, that's the only way to buy without regretting it.
P.S i have to disagree with member luvchnl, i saw a few pictures of luvchnl wearing the Chanel 5102 sunglasses and i fell in love with her photos. She is the reason i bought the Chanel 5102 as they looked so good on her. I normally wear really oversized sunglasses but these where smaller and now I love mine. I think the logo is perfect, not too big not too small. Shame she doesn't like them though. Just goes to show you that individual taste varies so much.