ThunderLord's Decree or DFT?

So I've been using Lissandra for roughly 2 months now. (1 S+ away from M7) I have only used Thunderlord's Decree on her for the burst damage assist it gives me, almost adding another ability to my arsenal for extreme burst damage. However Lissandra is both burst and sustained damage. So I'm facing an inner battle. Which will serve me better?

Lissandra's kit has 2 AOE attacks, her W(Ring of Frost), and her Ult(Frozen Tomb). But what is her Q(Ice Shard)? is that AOE as well? And would her E(Glacial Path) be considered AOE? If so that halves the duration of DFT.

I'm thinking I may want to try DFT because I'm entirely ability reliant. I also have a very patient play style in lane. I believe that adding the DOT damage to Lissandra may make securing kills easier without having to worry about a 30 second cooldown early game when flash healing is common and often costs me the first blood.

Can anyone who plays lissandra provide any insight to this?

(Let's not forget that I will also gain 7% MPEN from the mastery Piercing Thoughts before DFT which I would take over the CDR of Intelligence.