I can't speak from first hand experience, but I work for a company that uses both. Mastercam is used for basic 2D machining - pockets, holes, c'bores, etc. Creo manufacturing is used for complex 3D milling. I know that Mastercam can do a lot of what Creo does, but not with the same ease. My guess is that if you are just getting started then Mastercam would serve you well - especially if you won't be doing a lot of 3D contour milling.
Creo touts the ability to regenerate programs easily if the model changes. I have very rarely seen this be an advantage because smart people don't start building something until the design is finalized. The only time savings here is if you are in such a hurry that you don't have time to generate toolpaths after the design is finalized - then you could prepare the toolpaths in advance and then just regenerate when changes are made. It doesn't take that long to prepare enough programs to keep the machines busy while you are creating the rest of them so the real time savings are insignificant.
I don't know what the cost savings are, but I'll bet they are substantial. If I were you, I'd start with Mastercam and see how it does. In the future, if you find it doesn't meet all your needs, you could buy a seat of Creo Man and use them both in parallel. This would be a good approach since you are new.