Probably the general rule is how technical you want it to be. I am a fourth year BIT, and I think the BIT and ISM program is becoming more and more similar. I myself was considering between Comp Sci and BIT at the beginning and choose BIT because I don't want to know too much of the nitty gritty of algorithms, but I do want to know how to code some of the cool stuff in computing.
BIT has electives that you can choose within computing, where as ISM has electives that you can choose within business. Both of them are heavy on INFS subject (doh) as the degree comes from that school.
In terms of how many electives, afaik ISM has more electives in comparison to BIT.