Well, I can really only go by what I've seen with Glory. We never really get any sort of full scope as to what she's capable of beyond her human form.
That being said, I think that a pre-"Time Bomb" Illyria would decimate Glory. They're both incredibly fast, but Illyria is so fast she can borderline control time. I think they're both equal in the strength department. The only big difference I can think of is that Buffy was eventually able to beat Glory straight up, while a fully powered Illyria absolutely wiped the floor with Angel, Spike, Gunn, Wes and everyone at W&H.
I think that before "Time Bomb", Illyria is the single most powerful character we see in the entire Buffyverse, bar none.