The Printrbot Simple (wood) has a bit of an issue with the stability of the Z axis; this is one of the first things most users set out to upgrade. (And the new Metal version of the Printrbot Simple specifically addresses the issue with a redesign of the Z-axis rods -- increasing them from 8mm to 12mm.) The Prusa I3 will probably cost you a bit more, but in my view, it holds the advantage. With that said, either one is a pretty good choice for a first build.