Don't quote me, but AFAIK molten armour gives you 35% spirit into crit raiting, and then you get a glyph which gives you 20% spirit into crit raiting too, so overal that 55% of your spirit to crit raiting ye?
So 18 spirit = 9.9 crit strike raiting, where as Icewalker gives 12 crit strike raiting. Now granted that spirit does give you mp5. But atm MP5 doesnt give you dps and you shouldnt be going oom otherwise you need to look at your mana conservation in fights... So imo Icewalker > 18 spirit, as eventualy with the extra hit you get the chance of reducing hit elsewhere, either through gems or gear for alternative dps gems/gear. Like imo id change your gem in chest for SP/Crit or summit.
Like i say i don't play a mage so the maths maybe not accurate you could do with looking into it..