Hi,
Firstly, you should mention your use case and which i3 or i5 model you plan to go for. Also, what is your budget? i3s are dual core processors with Hyper Threading whereas(2 cores, 4 threads) whereas i5s are quad core (4 real cores).
If you plan to buy this for gaming, I would suggest go with i5 as most modern games take advantage of 4 cores. I would also suggest you to buy 270 chipset motherboards.
Review of i3 7350K: http://www.pcgamer.com/intel-core-i3-7350k-review/