The only difference I can see is that one foregrounds its thematic ambitions in its recasting of popular genre material, and the other is content (like, it must be said, the books themselves) to shape itself as a plot-driven entertainment.
To elaborate on that, one of the things that most disappoints me about the TV show is not a change but the absence of one. I had expected, based on the way Benioff and Weiss pitched the series, that they were planning to do for fantasy as a form what other HBO dramas had done for mob stories, cop shows, westerns, etc: take the building blocks and do something artful and ground-breaking with them. That would have involved tweaking the source material, cutting away background detail that's charming for readers of doorstopper fantasy but inessential, and changing the emphasis so that stuff Martin leaves unstated comes to the fore. In such a version, scripts by Martin himself would have stuck out as weak points, because he's not a screenwriter in that league. Instead, they've been faithful to the narrative complexity, but played it as gritty entertainment, like a glossy, well-cast version of The Tudors
. Which makes for great TV of its kind, but more was possible.