Computational science is nothing like computer science - computational science is basically just learning how to make a computer do things for you (eg, calculations), whereas computer science you'll actually learn about computers.
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Computational Sci is a subset of Computer Science. Computer Science allows you to take units from the more generalised information technology degree such as Data Management, Systems Development where as Computational Science doesn't, it just deals with that - Computational Science. In my view, Computational Science is the difficult part of IT, it has EXTREMELY heavy ties with high level mathematics and 'being good with computers' doesn't cut it. You'll study how computers/programs/algorithms/data structures work at an abstract level.
Fortunately, few businesses see the difference in Computational Science vs Computer Science, so with either Computer Science or the Computational Science major you should be set! I'd say, unless you 100% sure you have a deep passion for Computer Science take Computational Science within your science degree.