Both are gems. BG2 is a better game and P:T has better writing and art.
I don’t think this is as cut & dry. BG2 and P:T have different objective with their writing - P:T has definitely more depth and finesse but it would be detrimental to BG2 if applied there. BGs are very bioware, black&white pulpy adventures, while P:T does something more thoughtful.
I think we can see the effect of merging the two in Pillars Of Eternity series - which tried to pay a homage to IE as a whole structure and adventure of BGs, combat of IWDs and reflective, subversive storytelling of Planescape. While I love the series, those three games combined into one clash rather hard at times.
