Best choice: i3 7100 or i5 4400?

It's a tough call...

- the i3 is 4 years newer

- the i5 has 4 cores vs. 2, but the i3 has hyperthreading (meaning it can handle up to 4 tasks)

- the i5 has double the cache of the i3

- the i3 has a slightly higher clock speed (3.9GHz vs 3.3)

- the i3 supports more / faster RAM

I'd go with the i3 because it achieves a similar result at a lower power draw (51W vs 84W).