What I reckon...
I'm gonna go with ems.
Because as far as users go, everyone is different... and legibility is completely subjective.
If someone has gone to the trouble of creating or having a website created, they've obviously got something to say. That means words are vitally important & there is nothing more disappointing & frustrating to a user than crucial text not being legible.
Lets not forget, there's old folks out there that can't see what's on their dinner plate let alone what some designer has fixed at a squintworthy sizing for the sake of aesthetics.
Sure, people appreciate cool design, but it has to be subservient to content & usability and let it complement not distract from content.
So I reckon, make your font size a em / percentage.. and ensure your user can appreciate your design at same time as get what they've come for in the first place...