How to choose materia for each class?

[Taika]

I've heard spellspeed and crit for BLMs. For my SMN spellspeed is the biggest (can increase the amount of ticks in a DoT), and Crit/Det should be roughly equal, Since Crit Ruin III is nice, but Det adds so much to DoT and pet damage that I really don't understand the SMNs that can advocate a purely Crit build... :/

Actually, for SMN, the tested secondary stat weights are CRIT (0.287) > SpS (0.195) > DET (0.192).

If you use Ariyala's to compare different gear pieces and sets you can compare their stat weights (BiS solver --> Show stat weights).