Just my opinion but I would do the Lapras. Defenders no longer really matter in most areas. Hell you need someone to take you down and whatever you throw in there will earn or not depending on most areas tbh.
So I would do the Lapras. Top tier attacker for a lv 4 Raid boss. A rock move Golem may have some uses in the future but a Lapras and their candies are much harder to get in almost any biome.
Edit to add: That Lapras would be a decent defender in the new system too.