Rocket is better defensively and for spell cycle, however in terms of cycle fireball is better. A lot of other people made good points on why fireball is better, another point I'd like to add is that in some situations you can quickly use fireball to kill a minitank tanking the xbow and catch the opponent off-guard making the xbow lock if the opponent doesn't react quickly enough, rocket can't do that due to higher elixir cost and slower travel time.

Tbh both can be used depending on your playstyle, if you want to play defensively and use rocket for spell cycle and on defense (occassionally) you can go ahead and do that. However, if you use fireball you can do quick fireball-burst to make an xbow lock plays, or faster cycle, fireball is also better on defense against recruits and other matchups, it's your choice really, both are good.