definitely ro/di water.
No point to having water in your tank at all (except to check for leaks) until your live rock gets here, so empty it out, dry it off, and wait for your rock.
I advise laying down some lighting grid (eggcrate) in the bottom to protect your glass, then your live rock, then the sand, then the water---and you have to mix the salt INTO the water before you let it touch your live rock or you will rapidly have dead rock. Plastic white painting buckets (polystyrene) and a maxijet 1200 pump are very useful.
There IS no debate about ro/di being good. There are probably about 4 places in the continental US where it's not necessary. Your odds of being in one of them are not that high. Go for the ro/di---spendy, but so is getting rid of the algae bloom that attends tap water.