They're both great preamps it seems. I think it's a matter of what you're looking for in terms of sound, and not even that one sounds better. They are just different. The Focusrite's are great sounding straightforward preamps. The ART's are great sounding tube-driven preamps.
Me, personally, I'm filling out my Saffire Pro 40 with the Octopre MkII via the ADAT, and the ART MPA II Digital via the S/PDIF.