Both types may be considered to be "purified." The process by which they purify is different. In the distillation process, water is boiled and turned to steam, then cooled to condense it back into a liquid. The result is very pure water.
The reverse osmosis process is simply a filtration method. Water is forced through a "semi-permeable" membrane. Only the smallest of molecules pass through the membrane. Some contaminants are smaller than water molecules and will pass through. So although some ro
water may be classified as "purified," it may have some contaminants left in it.
You should be able to use either type in your top-off - they are very similar in that they neither should have any major contaminants.