Originally Posted by
HolyNova
I've tested it and it seems that penance is bugged, that mean you won't get doom nova procs from it. I've reported this bug, because penance should interact with the trinket the same way arcane missiles do. But even the initial tick of penance doesn't trigger a doom nova for me (I've got the mythic version of the trinket).
Spamming SW:P works, but Smite should be more dps if the target already has SW:P up and running. Casting SW:P or Holy Fire just before the doom nova debuff runs out will result in one additional proc, however.