Kold Steril (KS) is NOT even in the same ballpark as RO/DI. KS is very good filtered water, but not pure by any means. KS will knock out the chlorine, and some other nasties like copper, but will let nitrates and phosphates pass through unchecked. But RO/DI is pure.
What you need for water though depends on your system - freshwater or salt? I run saltwater and RO/DI is pretty common. Freshwater... I get the feel that most folks just use dechlorinated tap.
I started my SW tank using a Kold Steril system for the reasons you noted - no waste water, it doesn't strip the water of the "good" things like calcium, alkalinity, etc. But there's a lot of bad things it doesn't strip either. When I noticed a sudden increase in nitrates in a quarantine tank, I eventually traced it back to my source water. When I originally made the decision to go with the KS system, my tap water tested zero for nitrates and phosphates. Months down the road, that wasn't the case anymore. At that point, the light went on in my head that water quality isn't really a static thing that never changes. It can vary month to month, week to week. And what things are in the water that I can't test for that may effect my reef? At that point I realized that consistancy is what is best for my tank, and the only way to get it was to start with "pure" water.
Long story short, I plumbed a dual cannister DI system (no RO) after my KS system. I then plumbed the outlet of the DI to a separate faucet that is mounted above my laundry tub. I can fill buckets with pure DI (0 TDS) water on demand, with no waste. No need to store the pure water, but I do have a storage tub for the made up saltwater. I can flow through the DI resin at 40 gal/hr and still get 0 TDS output.
BUT... the system ain't cheap. Since I have no RO membrane to do the heavy work of pulling out all the junk, the DI resin depletes fairly fast. I go through one cannister of DI resin every 6-8 weeks. If you budget allows it, it's a fairly easy no-fuss way of getting pure water with no waste. But I'll be the first to say it ain't cheap. But you always pay for convenience no matter what it is you're buying...
The KS makes an excellent "pre-filter" for the DI system, and it does pull junk out of the water. After a year, my KS pads are a nice bright blue color - showing that it's adsorbing copper out of the incoming water. I change the KS pads/carbon block yearly - in fact I have a spare set ready to go in next week!
Hope that helps you decide. Don't make the mistake I made and think you're getting nearly pure water from the KS. You aren't. But whether you need RO/DI or not... well... that's something you're going to have to weigh the pros/cons for your own situation.
