I vote for the RO/DI unit and throw in an auto topoff unit too. The closest you can match evaporation, the least amount of salinity swing you will get. Salinity swings can be VERY damaging to some of the animals we keep in our tanks.
You CAN do an auto topoff though and still buy your water (salt and fresh) from your LFS. Auto topoff units are very inexpensive. But then again, RO/DI units aren't outrageously priced either.
