That one is a Commerce major more than a Science major. At least three of the subjects for the major (ETC2440, ETC2520, ETC3510) are basically watered down versions of Science units and if you take the MTH versions then you wouldn't be able to enrol in the ETC ones.
I don't know about career prospects but I would assume there's no advantage to that at all over a proper Prob/Stat major with the third year actuarial subjects on the side (if you really wanted to). You could PM TrueTears for more info, if anybody around here would know, he would.
The major you are referring to is Major in Financial and Insurance Mathematics offered by the Faculty of Business and Economics, which is indeed a Commerce major. The one OP is referring is Double Major in Financial and Insurance Mathematics, same name, but is a branch of Mathematics and Mathematical Statistics major. Some of the core units are MTH2222, MTH2232, MTH3230, MTH3241, MTH3251, ETC3400, ETC3460, ETC3420 and ETC3530.
Back to the question, I haven't really done any of the above units myself but I'm interested in this major as well. To me this major looks like a fantastic combination of Mathematical Statistics and Applied Statistics/Econometrics. The ETC units allow you to work in the finance sector (I'm not sure why exactly you are qualified after completing this major though). The MTH units prepare you for further study in Mathematical Statistics/Stochastic Processes/Financial Mathematics, but can also help you make sense of whatever you do when you work in the finance sector.
And as mentioned, TrueTears would know everything about this.