Older music has a different style than newer music, for sure. More ambiance, a lot simpler in nature; yet a lot more memorable as well (and not just for nostalgic reasons). A lot of more current music - WoD, Legion, and BfA in particular - has more thrilling, battle-themed motifs to go along with some of the more ambient tracks, and that's perfectly fine, because it makes sense to have that playing during important events and encounters.

Overall, I like both older and newer music - older because it's the sort of music that stays with for years after years and hearing it again sends you right back into Azeroth even after so much time away from it, and newer because the overall quality of it has stayed the same despite the style and themes changing and improving throughout the years.