For me it depends level to level. The harder a level is, the more I prioritise gameplay playability and general integration with the rest of the level (I suppose this would be considered a more holistic outlook), up to roughly 50/50 for extremes, but it really depends on what the level's trying to do conceptually and how it actually plays/looks. By and large I'd say I consider decoration more important, but really what matters most to me is all the elements share the same vision and they all work off each other well