It aallllll depends. Both have strengths and weaknesses.
I started with pepakura in 2010 and made a whole lot of things from weapons to helmets and armor and all kinds of things. It gets really tedious with all the folds and details, but if you take your time and go slow the results are great. Harden it with resin, add some fiberglass, and you will NEED to round it out with bondo. Pepakura is a very digital thing, and as such cannot do round things very well, so you'll need to compensate with the body filler. You can also look up 'rondo', and that works for getting into hard to reach places as well.
I started last year working in Foam and I haven't looked back. It has its own set of tediousness and challenges (bevel cuts *shudder*) but you can get really good results with foam, too. It also tends to be a little bit cheaper, but doesn't stand up to punishment like fiberglass does.
I would strongly suggest trying a piece like a shin or forearm in both pepakura and foam and see which one you like better. Watch videos on YouTube from Evil Ted and punished props and they will guide you down the right path. Andrew DFT has some really good tutorials as well, but if you follow him strictly, you could develop bad habits, since his cuts tend to be a tiny bit more choppy.
I would direct you away from doing helmets or weapons right at the start, simply because they are more advanced than you'd realize. But once you have the hang of it, the world is your oyster.
If it's worth anything, I find helmets to be better in pepakura and fiberglass than foam... foam makes them look bubbly sometimes.
It should go without saying, though, that the biggest requirement for either is you need to keep this forum updated with your progress. Common courtesy, doncha know.
Good luck, and keep us posted!
