Milliseconds are a measurment of time. Ticks are a measurement of game logic/rendering.
In other words, a tick is a frame. If your game is overloaded with lots of nested loops and heavy use of shaders on an old computer, then your FPS (tick rate) could drop very low, but milliseconds will still keep going on by like normal.
So you can't really reliably gauge time based on ticks. If your game is 60fps and you say "Every 60 ticks" then you might think you're going to activate that event once per second... but what if your game drops framerate? If your event is time sensitive then you're boned.
Generally speaking, if you need something to happen in a specific amount of time, use milliseconds. I hardly ever use ticks, in fact I can't even remember the last time I did.