So since no one has answered your original question, here is my take on the comparison.
So the main reason to use RO/DI water is basically to have nothing extra in the water other than what you personally add to the water. It is removing variables that can cause problems in a reef tank. The main things that we use these water filtration methods is to remove heavy metals, chlorine/chlorimines, phosphates, nitrates, nitrites, ammonia, silicates, and misc organics from the water. These all can have negative impacts on the tank. Anything from being toxic to your tank inhabitants to causing heavy growth of nuisance algae.
And as for cost, it is much cheaper in the long run to buy an RO/DI unit than it is to buy water from a store. Out here in D.C., they charge $1 per gallon for RO/DI water and when you add it up, 150g and you have paid for an RO/DI unit. And if you only mix 5 -10 gallons of RO/DI your filters will probably last a year or 2 if you properly swap your sediment filter and flush your RO membrane once a month. If you also have to drive an hour to the fish store, that adds even more to the cost.
So as the others have said there is no pro to using tap water because the money you save using tap water, you will have to spend on more cleaner crew to handle the algae and pulling out the unwanted heavy metals out of the tank. And even possibly might cause fish death if the heavy metals and ammonia levels get too high.
