6 determination = 1 mind.

Not sure what's the spell speed calculation, but for us scholar, I would take crit>spell speed > determination.

For whm though, I think I would take determination over spell speed as I'm usually comfortable at healing and that bit more heal squeezed over long run can make our healing spells a bit more MP efficient, which seems like the problem of whm on general.

Don't get me wrong though, value wise I think spell speed will be larger than determination, it was just my play style that makes me pick det over speed as whm. If I'm not mistaken, every point of skill speed is equal roughly to 0.1% casting speed increase.