Pentoo comes in many flavors and it is important to choose wisely.

Right now, you have two main choices:

Choice 1: Full or not (you do not want Core)

If you plan to install, get the Full iso.  If you want all the tools, get the
Full iso.  If you want to update anything and need the build time tools to do
that, get the Full iso.  Basically get the Full iso unless you are only
planning to run live and don't need any of the more rare tools.  If you are
reading this, you don't want the core iso.  No one wants the core iso, it has
no X, no security tools, and it basically only useful for kernel testing.

Choice 2: x86_64 or i686

Reality is, no one tests things on x86 32 bit (i686)  anymore.  I don't just mean us, I mean
software devs, distro devs, and even kernel devs.  No one cares about old
hardware.  If you download the i686 builds, it will run on anything Pentium M
or higher, but reality is that the x86_64 builds are not only faster, but
hugely better tested. Pick x86_64 unless you have a really good reason (like
wanting to support ancient x86 32 bit hardware).