There is no such bug on LK as a fire mage who has no spirit items and is completely BiS(referring to faelarin's bis list) with proper SP gemming you're going to have ~75% give or take.So w/e you do STATISTICALLY you will have 3/4 crits on all fire spells,a crit stacked mage will have 4/5.And the reason why your guildie is doing more is probably because he knows how to play,Many BiS mages can pull dps but few can pull big dps.But really the only way to get an accurate notion of which is better crit vs sp is by playing each one and talking to people who have played them,go ask Faelarin,petkobika(ragnaros,aurora),myph(ragnaros, radiance),my gm Banebane(deathwing,encore),purl/leulz(ragnaros,aurora) if they still play,Xego(ragnaros,aurora),copper(i believe he's in ragnaros,radiance),there are other mages but i'm more aquainted with the ones on Ragnaros