I'm certain that spell speed only wins in a perfect scenario, where you basicly auto cast whenever possible. If you consider moving around, input lag and even human failure, DET and CRIT should come out ahead.

That being said BiS should be:

Allagan weapon+shield, left side full allagan, right side earring and 1 ring allagan, rest hero's (119SS, 135CRIT, 66DET)

more accessible should be:

Relic+1, AF2 head, chest, hands, Allagan waist, legs, feet, right side like the other set (152SS, 150CRIT, 34DET)

Feet are interchangable with AF2 of course

Earring could be argued about, but going by stat weights 16 crit > 8 det (as long as you are not at the 15% cap)