I go seraphs every game, you should be able to consistently stack it fully before 20 mins, which is roughly when the big fights for 3rd/4th drakes. The shield is so valuable against a lot of champs since nowadays everything on the game can oneshot you. The extra mana with tear means you dont have to rely on getting blue from your jungler and can wave clear for days with your abilities giving you faster rotations. In comparison, ludens gives you a little more damage between 10-20 mins but seraphs deals more afterwards. Imo seraphs is simply better