Technically, the Ryzen 5 1600 is better, or should be better, but according to many calculations, the Ryzen 5 1600 can be 5% better, to 10% worse. The i3-8100 is stronger for gaming but the Ryzen 5 1600 is a more powerful everyday, workstation cpu.
This isn't the only thing to consider though, as the Ryzen 5 1600 also comes with a much stronger cpu cooling fan than Intel's stock cooler, but it's around $100 AUD more ($70 USD or something).
Overall, I would choose the i3-8100 but if I had a budget a bit over $1200 AUD ($850 USD or something) I'd prefer the i3-8350K for much stronger performance in every category against both the i3-8100 and the Ryzen 5 1600 by around 15% - 30%.