Ceramic or porcelain floor tiles for upstairs bathroom

I would go for porcelain, it's a lot more durable than ceramic

Obviously, depending on the existing floor type, you will need to put down a durabase matting to stabilise a wooden floor, but, as to the type of tile to use, as it is not a heavy traffic area, either porcelain or ceramic tile would be fine, with the porcelain tile being the stronger of the two

When tiling on a wooden floor you lay cement boards with adhesive and screws which locks the floor preventing any movement then you can lay any tile you like.

I would go for porcelain. Although harder to cut they are much harder wearing than ceramic. Make sure you choose the right adhesive depending on the substrate ( surface) you have for tiles to go on ie, wooden or concrete/cement.