Dusk shield is a really powerful shield, effectively cutting the damage you receive in half. Sadly, at it's cost of 6 darkness, it is relatively expensive for a shield. Upgraded however, it's cost becomes a mere 2/3 of what it used to be, now only 4 quanta. Improved Dusk is obviously much more affordable and efficient. While Dusk Mantle is a very good shield, it is often ignored in favor of Fog Shield, whose cost is one third Dusk Mantle's and only suffers a 10% miss chance. As a general rule, unless you're focusing on a darkness heavy deck, Dusk Mantle probably won't serve you to the best of its abilities as if you were using a deck that fully utilized darkness quanta.