You can snag the Instant Fortress power and some meh heavy armor from an optional boss on Varena's route. Aside from that, it's more a question of who you want to do the dungeon with.
Calise generates a shitload of aggro simply by existing, so she's going to be the tank no matter what you do. Varena meanwhile is a pretty solid storm damage caster. Additionally, you fight Blood Knight alone and then go straight to Chaos Blade in Kass's route, whereas in Varena's route you have to fight Blood Knight and Faceless Blade, then Chaos Blade.
You also have the option of attempting to betray and kill Kass on her route, but that's a dead end for now because Savin said winning that fight will take you straight to the game's ending once it's implemented, which won't be for a while I imagine.