Well, spotting your enemies is pretty important, but I could never decide on which I wanted to use. Sure, monster events allow you to have bonuses like advantages, etc, but it's very easy for the player to just avoid all those fights and head straight to the boss underlevelled. Random encounters are well...random, and can be annoying, but they're also a lot easier to implement, and save you the work of having to make enemy overworld sprites...
What's your say?