Hmm, have you taken a look around yet to learn a little bit about each process? Better do that....
Anyway, a lot of factors should be considered when determining your method:
How much are you willing to spend?
Do you have a lot of time, or do you want to finish it quickly?
Are you good/knowledgable with sculpting?
Do you know how to use fiberglass resin?
How many suits are you planning on making?
How sturdy do you want it?
I know....lots of questions.
Basically, pepakura will give you a 100% accurate ratio between the pieces, and will fit right if you scaled the whole thing correctly. It also happens to be the cheaper way of making armor, and looks kinda good too. the only thing it fails in is getting smooth curves, but that can be negated with enough patience and effort.
Modeling out of clay gives you the opportunity to enhance your armor with tons of detail, but requires many hours and lots of clay....not to mention skill. then comes the pricey molding and casting process, but the benefit is that the more suits you make....the cheaper your molds become.