Roster
Depends on your roster.
Scarlett
I tend towards diversity first, then duplicates, but your play style may be different.
So with no information, another red 4* before a second Gormek.
Colen may be the only exception among red Classic 4* heroes, but Colen is another rant.
See also
Scarlett and Jackal discussion
(Two Rogues or comparing Guardian Jackal and Scarlett ( with a mention of Grimm ))
Discusses many red 4* heroes
(Kelile emblems? - #16 by Gryphonknight)
2x red defense debuffers versus titans and triple bosses
(Wu Kong Debuff Duo or Smart Probability or Two Defense Down heroes with Wu Kong)