Little known bug but whatever item is first procs spellblade. So if you look at your items, and have Divine Sunderer and then Iceborn, DS will proc Spellblade but Iceborn won't unless specific circumstances - like Iceborn will proc a patch of ice if you're fighting minions but DS will proc if you're hitting a champion.

You should only take one Spellblade item if you don't require the stats from multiple Spellblade items which is super super super rare.

In my experience, Divine Sunderer is better against tanks. Iceborn is better against anyone else. Your own lane matchup decides what to take. If I'm Wukong or Nasus I take DS to capitalize on the empowered Q. If I'm any other beef baby or tanky boi, I take Iceborn.