The difference seems to be the level of chashivus given to the non-Torah portion of the equation. Torah im derech eretz has Torah – along with derech eretz (secular studies). Torah u’mada has Torah and science – both of them equal or near-equal.
This is not the way all followers of YU view it, and there is definite overlap between the two philosophies – but that’s a simplified description of the differences.
an Israeli Yid