Though Illyria and Cassie are, for the most part, on par with one another in strength and durability--especially following the ability boost given to Cassie on Ares' behalf(post Infinite Crisis; OYL)--I'd have to cast my lot with Illyria. Is it merely Whedon-bias that motivates my opinion, you ask?
, no.
My decision in this conflict is ultimately based off of an analysis of the two combatants respective capabilities. Consider: both Cassie and Illyria display phenomenal levels of physical--albeit mystically enhanced--strength. Cassie has picked up her share of vehicles and heavy structures just as Illyria has gone toe-to-toe with two powerful vampires...armed vampires, I should add. In terms of resillence, Cassie has on one occassion tackled the newest Post-Crisis Supergirl and lived to tell the tale, even after sustaining massive attack and concussive force. Similiarly, Illyria has been pummeled by multiple mid-to-high level demons only to emerge without the slightest injury. I could go on and on about the commonalities in their ability set, but I'll get to the specific ability that woudl bring Illyria victory in this conflict: time warping.
Illyria, before being Byrne'd (Superman fans, rejoice), had control over time itself. She was able to twist the flow of time to her will and specific intent which served to slow her opponents down to virtually no motion. As they made a single stride within a two hour time interval, Illyria had already landed at least sixteen consecutive blows! Cassie can't compete with that kind of power.
