The Old Way when Orbs didn't exist--
Situation 1: Enemy doesn't have SR--Optimally you would cast an Evocation, since they do much more damage than the Conjuration equivalents and they do it right now. You have to prepare some Evocations so that you can do this though. If you're stuck with only Conjurations you're in trouble.
Situation 2: Enemy has SR--Optimally you would cast a Conjuration spell that ignores SR, but if you're any good at caster level, the Evocation with the SR chance actually looks attractive here too, but either way, the presence of SR lowers your damage output.