Completely different ranges. The HTDM150 is a old discontinued model and the 300 is a current model. The HTDM550 was the upmarket higher power version of the HTDM150 but it simply didn't sound as good and I because of its greater power fan noise was more of an issue.
I know its not a definite indication of sound quality but look at the size of the speakers with the 300 model and then look at those with the 150. For a start the centre speaker with the 150 is about a foot long, heavy and fully shielded.