Soundtrack for TT was indeed excellent. Not only were the songs good, but they fit in with the series' atmosphere damn well. Examples:
Spoiler for Save space:
Nice song that sums up the show's overall atmosphere:
Arguably the most well-known song. Perfect example of a theme song that fits a character's personality
:
And my personal favourite:
So yeh, HSI pales in comparison to TT's soundtrack. However, I wouldn't go as far and say HSI's OST is terrible or anything, just maybe not really noticeable. Then again, there are only a handful of OSTs that I would actually go and listen outside of watching anime - mostly Tenmon or Yuki Kajiara.