personally would go for an i5 over the i3. The cost will be pretty negligible. I bought an acer i5 6gb of ram for just under £400 a couple of years ago. I asked an advisor in PCworld and they said ‘all i5’s are quad core’. Looked it up myself and he was talking rubbish, only the desktop cpu’s are quad core the mobiles are hyperthreading dual cores. But they are still pretty fast.
I might be wrong but coding wise you don’t need much power. Processor comes into play when you start using things like animation programs and photoshop etc. I have an i3 for work which is about 5 years old and its fine with adobe cs5.5 suite, just had to add more ram so i think i now have 6gb. I could happily code websites on a pentium 3 with 512mb running XP if i needed to.
When i did some 3d animation using blender it would run the cpu at 100% for hours on end but there was always enough ram (8gb on my desktop).