None of the currently made iMacs have an i3 processor, so your question lacks a lot of detail to get a good answer.
What exact i3 and i5 models are you talking about? The i5 of the same model year as the i3 system will in general be faster. There may also be graphics differences and other differences between the two systems that will change the answer as to which iMac is better for you.
There isn't a 3.6 i3, there was a 3.6 dual i5 BTO 21". All of the new models are quad core, so you should see fairly decent gains regardless of the model you choose.