They are completely different beasts.
The DT990 is truly phenomenal, remaining one of the best all-rounders I've heard. That said, it's open so is terrible if there's ambient noise or if you need to worry about sound leakage. They are also heavily amp-dependent, and really shine with tubes. Also, the comfort is a huge plus.
The DT150 is also a solid all-rounder, but does have a slightly closed sound that tilts it away from acoustic music towards anything that benefits from that enclosed feel. They are less comfortable, but drive more easily, and don't seem to change as much from amp to amp.