I was under the impression Relics lost most of their benefits by being offhanded, so no damage procs/access to Mercy Stroke lead me to believe that the newer Daggers would be better. that, however, is just my gut feeling and in no way based on the maths found in the spreadsheet.
Correct, the OD3 and access to Mercy Stroke do not persist in the offhand, however, the attack, relatively high damage and low delay do.
That said, I don't know if it's still the best offhand for Excalibur, it never really was for Murgleis or Uson/Bura.