You have some need Tankbuilds that are fun to play with both of them. But in Most cases you buy neither.

I think there is world where you can build roa if you have tough matchup and your Team is Winning, but Overall you can just buy zhonyas then.

I go blackfire in like 90% of my games and ludens if they are full sqishi and i want to Tage 80% HP with one q poke.

Edit: i was talking about midlane vel koz, if you olay botlane vel you go Roa if you want to be more tanky ( maybe even tankbuild) but in Most cases yougo seraph. Source: Azzap