i'm sure this varies from school to school. at my school:
medicine - most hours, most reading, most staying up all night, most brainpower.
ob/gyn - also a lot of hours, less brain power required during those hours. less responsibility
surgery - too early, lots of hours, tons of scut, much pimping, very exhausting.
family medicine - easy 9-5 paradise
psych - easy 9-3 paradise (depending on where placed)
neuro - average medicine and psych, and you have neuro
pediatrics - easier than medicine but harder than neuro.
as a gauge of time constraints/pain levels, i will say that i was able to go running 4-5x/week in every rotation except for surgery (couldn't through any of it) and medicine (started out strong but lack of sleep caught up to me).
good luck. personally, i'm glad it's over.
--s