There are tons of FAQs around... short answer is that DI is much more complete in removing contaminates than RO is. You can run DI only, but it is more expensive. My office runs RO, and my TDS reads almost 40, while my RO/DI reads 2. Compare to city water (which I haven't read in a while) of well over 300.
That's a quantitative difference, but I can't tell you what the practical remifications to your reef are. They both run a carbon stage (or two), and that deals with the chlor(am)ine, so the most dangerous stuff for your reef is gone without RO or DI. My guess is that the traces of stuff left after RO is likely not too much to worry about.
Matthew
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