Originally Posted by aestropher
Maxed Static gives 6% more current HP in damage while maxed Arc gives 60. Therefore the breakeven point is 1k current HP considering Arc casts only.
Bolt has 3x the effective cd of Arc, so in theory the benefit of maxed Static does trigger 33% more often than that of maxed Arc; it helps a bit but diminishes as enemy HP gets lower during the course of a fight.
So, if your enemies are at around 1k HP, which is usually the case pre-level 11, go for max Arc.
Or feline friend here has given us a very good explaniation, however I want to add that while this might lead you to think that maxing arc is the ebst desition all the time, what our friend here is trying to point out is that the choice between those skills depends on the nature of the enemies you are facing. For example if the enemy has plenty of tanky heroes such has Treant, Jak, Batrider, Centa, Ogre Magi, etc. Then you should max Field, in the other hand if you face glass cannon heroes such has Sniper, Bone, CM you want to max Arc instead. Also take in to consideration heroes that fill those conditions but have skills that will make you rethink your choice, for example LS, Rubick, Void, Xin who will lead you to think Field or Arc are good choices but they have skills who provide them magic resistance in a %, or in a raw number (or magic inmunity) thus forcing you to take a different approach.