Personally on Nomad I see a lot of petards (quick way to win a castle fight, especially if they are virtually right on top of each other).
And I see siege towers used on arena.
Now watch Survivalist, and you will see both units.
On Arabia and everywhere 😀
Both are used as a quick way to get through the walls.
Petards are better becuse they desroy the wall and allow free movment of all units.
Siege tower dosent work on cav or siege so its not good alredy and you cant use it to get back out.
Petards are most commonly used on arena as a way to get through stone walls quickly if you had to build your castle at home insted of going for castle drop.
Other uses are if you are in castle vs castle war, 7 petards destroy enemy castle.
Siege towers are used only on arena for longsword rush or sicilian castle drop in enemy base.
Edit: 8 petards to take down a castle.