What is safer/better to drink tap water or RO water?

I work in a Marine Aquarium and we generate a supply of Reverse Osmosis water ~99% pure water. One of the staff says that he uses that water to drink because it doesn't have all the 'rubbish' that the waterboard adds. I can see where he is coming from but I understand that the human body is poor at absorbing pure water - hence isotonic waters with added salts and sugars. So which is better and safer to drink and would the RO water do any harm long term?

Many thanks

RO systems are widely used by waterboards to filter and purify water.  There is unlikely to be much difference in terms of safety between the water from your tap and the water you might generate yourself by an RO process.  As far as I'm aware most waterboards are not in the habit of routinely adding anything to the domestic water supply (in some parts of some countries flouride, for health reasons, is added to the water).  Largely waterboards are engaged in filtering, purifying water and removing things from water.  In Europe the 1998 EU drinking water directive defines the acceptable, strict levels drinking water quality.

Assuming your diet is balanced there shouldn't be much difference in absorption rates for RO water or tap water.   Isotonic waters, mostly marketed as "sports drinks", are notionally designed to replace lost salt when engaged in strenuous exercise.