I've just read the book after watching the TV programme.
I felt the emphasis was different and made them quite different. In the book, Connel rarely sees Marianne at school or sees interactions with those who bully her. I think in the book, you get a stronger sense that he actually does like her early on and Helen is more likeable as a character and their relationship more developed - he actually is happy with Helen until Robs suicide. Clearly we want him to be with Marianne, but Helen is a good character - they do love each other and it works for a good while. Peggy is more sinister in the book I think - more undermining and it becomes clear she was never really a good friend and the fact that there is a break up of the friendship group is more developed in the book. I agree that Connel is more aware of the class issue in the book - Marianne is a bit oblivious in both, but Connel's awareness of it comes through more in the book.
In both, I'm not sure the failure to communicate about the summer when Connel needs to stay at Mariannes really works, although in the book I think you get more of a sense of him gearing up to broach it and becoming increasingly unable to articulate it, so that when he does, it doesn't come out quite right. In the TV show, I think we see the scene twice - once through each persons view and I think the words we hear are not quite the same...to show they each think something different has been said. It is a poignant moment - a bit Hardy-esquire of failed communication which then diverts the story down a different route, even if it's doesn't feel quite right.
In the book, I think Connel realises earlier that no-one cared about him and Marianne and that a lot of it was in his own head. In the TV series when he speaks to her about people knowing about them, when they are at Trinity, I felt that even then, his tone says that he still can't quite believe they didn't care about it....he still isn't really beyond caring what people think about him and Mariannne in the home setting.
And sex is mentioned in the book, but there are very few descriptive scenes.....nothing about the loss of virginity for example. But those are good scenes in the TV programme and do add to it.
Unusually I actually liked the TV better.