none of them are better than the other.
both are used in woe to great effect.
scream is to force players to go high vit in woe, and increase your own ad firepower effectiveness, while also disabling lower vit classes, and not allowing classes built for damage to be too rampant.
FJ is for disabling people who actually tag to GR/AR/TGK in woe. spam FJ at the same rate as scream and then even fast switchers will think twice about tagging to GR.
a way u can make ur scream stun "longer" is to improve the rate at which u can scream. get higher aspd, higher int poem/-cast delay eq, and faster fingers. that way u will be able to stun enemies for longer durations, or even stunlock some.
stun and freeze, again, are for different people in different situations. stun a GR wearer and u still can't asura-kill him. freeze him, then u can. freeze a fast potter and he will pot right back after u break the ice. stunlock him, and u can kill him.
good players will not be immune to both, because there are times u need that low vit hwiz for maximum damage output in woe, same as u need people who don't just default ED/marc, and actually tag to GR/TGK.