metal's better for combat. because it relies on more on adrenaline, a key chemical for battle.
classical is better for epic, grand moments, because of it's ability to add emotion and give a sense of scope to whatever's going on.
two examples:
1. Megaman X has great fighting music and is nearly relentless combat
2. Morrowind had the epic landscapes, stories, and intermittent fighting. Great for a minimalist classical score.
one more example of a blend:
Halo, combined these two types very well, giving a grand feeling of scale (especially with the gregorian chant sections), but transitioned greatly to rock feeling when fighting intensified.