Welcome to the 405th.
If you're looking at doing the whole suit, do a smaller piece like the leg armor that Nogood suggested, or something like a forearm or hand plate so you get a feel of how things will work for you. That and if you mess it up, you don't have as much to rebuild. . . . . LOL! It's different for everyone based on your ability, but that's where I'd start. Also, if you're just starting out. . . . try to find some SD or low definition pieces to build just so you get a feel for the pepping or foam work, then you can move onto the more advanced pieces with more detail like your helmet and final armor pieces you plan on using.
As far as foam being easier. . . . I'll agree as far as putting it together. I don't agree with the prep work though. You have to make sure the parts are cut out exactly to make sure you don't have unsightly gaps that have to be fixed later. Just personal preference on which you choose though. Both have their pros and cons.
Hope this helps. And good luck with your build.