Looking at the special materials section, it looks like a Darkwood shield is substantially better than a mithral one. Both have a 0 armor-check penalty, and no Max-Dex bonus. But is there something I'm missing?
Darkwood Pros:
Much cheaper (257gp vs. 1020gp for a heavy shield)
2.5 pounds lighter
Resistant to rust monsters
Mithral Pros:
Treated as silver against DR
More durable
Cannot burn (but I'm not exactly sure if/when that would have an in-game effect...)