Looking at statistics, which shield is better? Heavy plate or mighty defender?
Shields
If its your first shield, I'd go for grey owlite shield over either personally. It's probably the most useful "standard shield" out there, due to its willingness to absorb bullets from gun puppies, its fire+shock resistance, and its fire + normal + elemental nature in general for the vanaduke fight.
If it isn't your first shield, and you're just looking for a good piercing shield, the new tortodrone shield is probably the greatest thing-- shock + poison + piercing is a great combination for the levels that grey owlite misses as well as the toxigel's spike throwing attack. The problem with aegis and ironmight is that both lack great status resistances. The problem with volcanic plate is that it lacks a specialized resistance of any sort- making it terrible for bullets and impractical for most melee attacks. Except for Vanaduke's mace, which it resists almost perfectly.
If you MUST get either plate or aegis then what Midnight-DJ said is accurate.
At the start of T3, mighty defender is better, as it can withstand specialized piercing attack. But later on, the heavy plate's upgrades are much superior to Aegis in every way due to they have stun defence and you will need the stun(and fire if you are going for volcanic plate) resist in FSC against the trolljans. In another word heavy plate has more potential of becoming a great shield than mighty defender, you can always make the defender into a heater shield.