There's another extensive thread on Ignites in this forum (
http://forums.elitistjerks.com/viewtopic.php?id=7672), but to quickly cite some stuff I saw there, to refresh the timer on Ignite to 4s remaining, you need to get a crit between the first and second ticks of Ignite (meaning in the 2s - 4s range). It isn't practical to land two scorches in the 2s-4s range all the time, so I think the math on keeping Ignite up is a bit more complex, even for a single scorch mage.
I think the original question is this: would a fire mage do more dps with very high crit or very high spell damage? I think you'll find that you'll maximize your dps by balancing the two rather than focusing on only one, due to the way item level is calculated and the dependence of crit and damage on one another in dps calculations.