I tend to like QoT more than Sedna, well at least when I'm playing with them. QoT can be an awesome auto-attack support DG as her ground spikes will end up debuffing 1500 armor from everything that gets hit with them, effectively doubling damage that all regular attacks deal. This works wonders with a creep wave hitting anything. QoT is also probably the longest overall ranged DG. Ground spikes will usually hit a few yards outside of its attack ring and Spike wave will always hit at least 5 yards outside of the attack ring and gains an additional 5 yards every skill level, ending up with an effective 35 or so range. Not to mention it will end up slowing 25% which is a huge amount.
As a pure general she is also rather powerful as her shamblers are ranged and have AoE attacks.
Shield is a wonderful anti snipe/hp buff but it very tricky to deal with since you don't get to see how many points it has left or how much longer it'll last.
Sedna I find normally just doesn't die if played well. But lacks the firepower that QoT brings with her. Sure Pounce does a huge 1k damage all at once, but is shrot ranged and can be predictable.
Overall if you don't want to die and want to try to keep your buddies alive, Sedna is the best choice. While QoT is normally better with fighting swarms of enemies, whether that be DGs or creeps or both.
