I think what jack's saying is that "font-size" to some extent is arbitrary. It's not scientific or worthwhile to compare fonts across a specific size if they have different x-heights. The only meaningful comparison is to evaluate legibility for fonts that are "relatively" the same size. For example, if Interface 12pt is equivalent in x-height to Roboto at 12.5pt, and Roboto is more legible, and legibility is the goal, then obviously the user should pick Roboto at 12.5pt (all things considered).