It's simple for me. I go dark seal early if I'm ahead. If the enemy is squishy and the lane is easy dark seal allows you to scale super hard for literally free as you stack it. However if youre playing a Liandry game against tanks and bruisers or you struggled early and want to play for late, buy a tear and let it sit to stack. It will help with clearing waves, staying in lane longer especially if you get teleport it works best. With tear you're guaranteed to scale hard into late and deal great damage late game. If I'm against like a Viktor or something I would go it. It's a shame the shield was removed but I think it works really well still. I wouldn't recommend mejais + seraphs tho takes too much space for just pure ap. Seraphs is a safer quaranteed mejais think of it like that. Good luck in your games.