Internet Science is kind of like a cross between computer science and electrical engineering. It's got a lot to do with how information is sent from one place to another through waves and signals, ofcourse it differs from major to major. This makes it more geared towards hardware and physical networking than anything IACT offers. I'm only first year, but from what I can tell IACT has a lot more writing/documentation/group work/projects involved, and deals more with things you would encounter in the business world. It's also a lot broader than BIST, you can do completely different subjects depending on your major, whereas BIST, at least for the first year, is mostly the same.
I've just changed from BIST (maj. Internet Technology) to IACT (maj. Network & systems management/Software engineering). Personally I think IACT will be a better degree to have to work in the business world of IT. If you want to make robots, design gadgets and work with networking cool things other than pcs, go with BIST, if you want to work in a business environment as a network admin or similar, go with IACT. If you find you don't like the one you choose to do, it's easy enough to switch. Because of the majors I was doing I was able to switch after semester 1 without losing any marks, but if you switch from BIST Internet Science to IACT Ecommerce or something, you may lose a lot of credit points and waste a year or more.