For frown lines, is Botox or fillers better?

At some point, thin wrinkles started appearing between my eyebrows, and it seems they are getting deeper and deeper as time goes by. Now, both horizontal and vertical wrinkles have formed.

I'm thinking of getting the procedure done now because I think it will be more difficult to treat if the wrinkles get deeper over time.

For frown lines, is Botox or fillers better? I have a feeling that Botox has fewer side effects and is easier to administer, but I'm still curious which is more effective for frown lines.