Well, even in t17 cavern if you have all of your worms on the bottom, you have to necessarily move them towards the ceiling to gain some territory even on the first part of the map. No one wants to leave worms at the bottom when SD is approaching, but when the upper part becomes too crowded, it's there that every move and position can become essential. Other type of skills are required and both have different kind of difficulties. It's, at this point, a matter of taste. I prefer T17 island: I can move more with parachute and rope and death by drowning is the latest of the thoughts, so it becomes more strategical than running towards the ceiling.