Is there really a big diff between RO and DI?
DI water isn't the same as distilled water or even RO water.
1.) Deionized water vs distilled water – DI water is as pure as the distilled water or even purer (oils etc.. can make it through the distillation process);
2.) Reverse osmosis water vs distilled water – RO water is more saturated with salts and oxygen than the distilled water and DI water;
3.) Reverse osmosis and deionization are a more cost-effective than the distillation.
There can be bacteria living in RO/DI water as it is not sterilized (like distilled water). UV sterilization tubes kill these bacteria (boiling wort and the process of making beer does the same).
Any of these water types will work for brewing as long as the appropriate salts are added back to the water.
The "cost" for me is justified by my reef tank. It really isn't a big cost because I buy the beads in bulk and repack the filters.
For brewing, RO water will suffice but it really is a matter of what you enjoy doing and perhaps your circumstance and environment. Which one makes the best beer? None. Making the beer is an exercise left to the user.