i3 Processor simply means that you have 2 processor that can perform hyperthreading task individually (i.e=> in common terms each processor can do two task almost simultaneously) so i3 processor behaves as if it have 4 processors embedde in it
On the other hand i5 processor really contains 4 processors each capable of doing single task at a time but very efficiently. And pf course i5 processor will be more efficient as it have 4 dedicated core for each task to do.
Talking about frequency 2.4GHz simply means that the processor as a whole can process or binary task (2.4 * 10 raised to 12 power) in one second & 1.8GHz similarly means that it can perform 1.8*10 raised to 12 power) processes in one second. So for the same processor configurartion higher frequency processor will do work fficiently.
But here by combing both the effects it will be decided which will perform task better & cant be predicted mathematically or logically.