To elaborate, Fire = 150 Potency and 2.5s Cast Time. Fire II = 100 Potency and 3s Cast Time. This means Fire is 60 P/s and Fire II is 33.3 P/s so casting Fire II on at least two mobs will be higher DPS than Fire (In the short run. MP cost will affect how much damage you do for an extended period). This changes at level 44 when you pick up the Firestarter trait which gives you a 40% to proc a free instant Fire III. At this point, you need to hit more mobs with Fire II to make it worthwhile.