Summary

Helios 300 2019 has slightly cooler temp due to higher fan speed (note that idle fan speed is audible unlike Y540 which is silent). Both of them have equally good cooling then. Note that Y540 is tested with 28WW BIOS which has slightly lower fan speed due to removal of max fan option.

Y540 is slightly faster in games with CPU UV+GPU OC (same with Helios 300 Turbo mode) with Optimus disabled. Slightly cheaper price too due to regular sales. If you prefer more metal build Y540 might not impress you though.

Credits to Jarrod'sTech