The S III doesn't sport a unibody design, so you can pry off the paper thin back cover at any time, and swap the memory card or the battery. The One X+ is shut tight, but comes with four times the 16 GB storage capacity of a basic S III.
The Galaxy S III sports a physical home key on a pretty thin lower bezel, which is comfortable to operate, and preferable for some people before the capacitive key found on the One X+. Most side and front buttons, physical or not, are easy to feel and responsive on the phones, with the exception of the volume rocker on the One X+, which is too flush with the surface, and with too shallow of a feedback, so it takes getting used to.