Mysterio will win because Loki lost a fight to Hela, Thor, Hulk, And Iron Man and the whole Avengers gain. Mysterio would have totally defeated Spider Man if it weren't for the fact he let one lousy drone get lost so Mysterio is going to win and Loki will get his butt kicked hard.
Mysterio isn't even real. He was using drones and stuff, And Loki would catch on quick. Even when the first Thor came out I liked Loki because if it were a real fight, Loki would kick Thor's ass. Mysterio doesn't even have a power. You're basically just fighting a technician with very dangerous drones under his command.
Loki (even if a weak one) is still a god with innate magical abilities. The mysterious at least in the MCU doesn't have any powers and relies solely on a team of engineers and cameras he stole from someone smarter than him.
Loki's power set includes; Superhuman strength, Speed, Durability, Longevity, Astral projection, Energy blasts and Flight. Sure Mysterio can appear to have these powers but at the end of the day he's just a good liar with a ton of drones