From what I've researched, Commerce Specialist allows you to go straight into the units related to your major. So, if you do Actuarial Science under Commerce Specialist, you will have to do minimal core units as part of your specialisation. You only have to do Economics, Stats and Accounting. Under the other specialisations there's even less core units to complete.
If you do a normal Bachelor of Commerce, you have to do six core units (Law, Economics, Acc, Stats, Management, Marketing) in order to fulfill the requirements for the degree. You have to do this regardless of what major you choose, even if you know you want to do a different major when you begin.
If you want to major in actuarial science, economics or finance, I'd go for the specialist option. You'll save time by avoiding some of the core units. If you do Commerce Specialist/Engineering, it allows you to complete your degree without overloading in two semesters. If it was available when I graduated I probably would've gone for it (Business Law core killed my GPA

).
Hope that helps!