I love both. I think that x marks the slots are best in the larger islands where you really need to use landmarks and topography to find them.
As for the riddles, I think they are the best, but most inefficient. I am not one for taking the most efficient route, so for me, I thoroughly enjoy scouring the island and communicating with my crew to figure things out. There is nothing better than discovery in a game like this and the riddles force that discovery on you. I ask a lot of, "Why is this thing here? What is its story" and you can genuinely piece those clues together. It is fun!