I think the discussion has a wrong focus to bring PS:T into comparison. The concern for only 5 companions is not whether their background stories or side quests will be rich or not, but whether this number can achieve the tactical flexibility and complexity of BG and IWD, which I think surpasses PS:T completely. If the players have no choice over party composition, then battle will just not be as rich as it stretches to be. This of course is built on the assumption that the party size is at least 5, which I think is also crucial in making the depth comparable to that of BG and IWD. The strongest point of PS:T is the deep story and dialog, never the combat system. Also, from what they announced there are only 5 classes, without any hint of whether there will be subclasses or not, which will also potentially make the party monotone.
Personally I will pay 30 or more gladly if they just use the IE to make an IWD3 instead of a game that only half way mimics the three great games. If budget is an issue, please focus on a few points and make them really strong.