Pressed vs. Rectified Tile
Another factor to consider when choosing your material is if you want a Pressed or Rectified tile. A simpler way to look at this is how tight do you want your grout joint? Some styles of porcelain are re-cut, also known as "Rectified", which means to correct the skew in the tile. Re-cutting the tile can only be performed on the four edges, not the top or bottom, as is done with natural stone. The re-cut is done by machines and therefore the tile is truer but still not perfectly square every time.
Through rectification, tiles can be set closer together as narrow as 1/8" (approx.) over a thinset application. Thinset is the normal installation method over a concrete slab. This method leaves the installation flat, not level, and is per the TCNA guidelines (Tile Council of North America).
When the tile is not rectified, a 3/16" (approx.) grout joint will be used per the TCNA guidelines. It is the same width used for all clay body tiles, and all grout joints are approximate with the subject of variation from piece to piece.