Moving up or down in divisions/weight class - which is more difficult?

[jas]

moving down in weight beyond a safe level is extremely dangerous for a fighter. it dehydrates a fighter to the point where the level of protective fluid around the brain is restricted exposing the brain to trauma. this fluid cant be replenished in the short period between the weigh-in and the fight. that is why there are safety weigh-ins weeks before a fight.

[-Kev-]

Fighters regularly go up 2-3 weight classes, some 4-5.

You will never see a fighter go down 2-3+ weight classes without seriously hurting himself. There's fighters that hurt themselves to make their own weight class.