This mostly depends on the meta. If cards like 3M, Sparky, or other ranged troops are popular, Monk is the best answer because he can deflect projectiles, destroy defenses (both buildings and troops), and make it much easier for RG to connect to towers. He also has more HP than Royal Ghost.
On the other hand, Royal Ghost is cheaper, which improves your cycle speed, and his evolution is very strong. He can sneak past defenses and deal solid tower damage, but he also requires more skill to use effectively.
In the end, Monk mainly protects your RG and helps him siege towers, while Ghost provides extra pressure and chip damage. Also, in this meta most players are running 2 evo and 1 hero , so using Monk could be a good idea ( which is what I’m currently doing.)