Is Illya worth the SQ in my case??

Xuanzang outclasses her for offensive casting so unless you want to add her to your card collection, no you don't need her and she probably isn't worth the quartz (again assuming you're not a collector)

Illya has the highest burst damage of all the ST caster to date but Sanzang has more consistent NP gain and doesn't suffer from a demerit on her NP.

As with most servants, only roll for them if you like them personally. For gameplay purposes, the only SSR servant you should ever concern yourself with is Merlin.