RO/DI unit imo for sure. A skimmer your tank can go without for a month if your just getting started, water changes without RO/DI can cause phosphates and other unwanted waste to get to concentrations that will absorb into rock and your sand bed from the tap water I am assuming your using now for water changes. Once this stuff absorbs it slowly releases providing a breeding ground for unwanted hair algae, and other types of unsightly algae that feed off the slow release of phosphates and silicates from the rock into the water column and subsequently used up by the hair algae to grow. This is only however if your tap water already has these things in it, some people use it without an RO/DI unit and they are fine. If you plan to do any type of saltwater tank a RO/DI unit should be your first purchase imo, it can save you from a world of head aches down the road.
