as far as I know, there's little difference between the two units - they are parallel subjects with same textbooks and similar style of assessments.
251 is relatively new, and introduced due to the high failure rate in accg250.
Under the double failure rule - failing a 200 lvl accg unit twice will exclude you from ur degree. so if u fail 250 now, u can do 251 instead of repeat 250, so that u'll have one extra attempt.
I heard that both units were easy for semester 1, I did accg250 in s1 and got thru without much trouble, although I've heard that both units became alot harder in semester 2, or at least the failure rates have gone up again.
so I guess that its doesn't really matter between the 2, different ppl may tell u that one of them is "easier" than the other one, although in the end it makes no real difference, at least the uni doesn't intend to create significant difference.
p.s. I dont know if this matters, SIBT students were encouraged to enrol into accg251 as they entered uni, but that can simply because that they need to boost up student enrolment in a new unit.
