Yeah, get searing cape for nukes, and get Goetia cape for sleep, 7 INT is atleast 3.5 macc allways, all the way up to 7 macc with the right dint(very unlikely, but still:P)
It's a nice cape for RDM nukes, but that's pretty much it
I believe neither Hecates or Ixion cape should be in your BLM macros, but to answer your question, depending on the overall gear setup, Hecates would beat Ixion.
Imo Twilight cape will become a fairly common cape for mages subbing /SCH