Honestly both have their pros & cons, For a beginner, foam is the obvious choice as it's easy to work with. However Foam does have it's drawbacks, no matter what you do, it will always look like foam & after long use, foam armor gets these creases & wrinkles in it & you eventually have to replace parts.
With Pep, you are able to get better detail, but it takes alot of work & alot patience. As has been said, Pep is the better option for helmets, however the hardest part of Pep is getting the sizing right, halogoddess has a great tutorial on sizing pep files & if you decide to go that route, you should definitely check it out.
In the end no one can decide which way will be better for you, you have find your own method...I suggest trying both methods & see which way works best for you.
Good luck!