So I'm of the ilk that loves BG:2, thinks PS:T is good, and like the other infinity engine games, but don't think they really quite compare to the first two mentioned.
The reason, I think, is that my RPG experience is defined by characters and stories.
More than the other IE games, these two games have a compelling/interesting overarching storyline. Perhaps judged on the storyline alone, PS:T wins, but that's kind of moot.
What I really think takes them beyond the others is the party interactions and characters, and they are the chief reason why BG:2 comes across as better the PS:T. (PS:T's aren't bad, they just don't really progress - by mid game, your party feels frozen, while in BGII stuff happens through the whole script almost.)
I really hope that Project Eternity has party members that have interesting back stories, have interesting quests, have interesting personalities, and who have their own opinions about things, sometimes strong enough to tell you no, but also, and, perhaps most importantly, can change because of your influence.
This is why BG:2 wins - it felt like you could have some sort of influence with your party beyond a canned romance. Being able to get Viconia to change alignments was a particularly nice touch.
I think a great world is nice. I think a great story is nice. But I think great characters that you get to have an influence-opinion-personality tug-of-war with are what make an RPG great.
So I'm really hoping the party members you can get are well developed interact with each other as well as the PC.