In general, it is better to use "Trap Stun" over "Forced Ceasefire" due to "Trap Stun's" lack of cost and stronger utility. However, "Forced Ceasefire" has one important advantage over "Trap Stun" in that it does not negate Trap Cards on the field that have already been activated – most notably, your own. Use this card if you need protection against enemy Traps but have a relevant Trap card in play that you do not want negated (such as "Safe Zone", "Metalmorph", "Skill Drain", "Pole Position", etc.).
Note that you cannot use this card in response to an opponent's Trap that has already been activated to stop it. As most Trap Cards are used during the Battle Phase, activate this card during your Main Phase 1 instead.