My RO/DI was a gift from a fellow reefer. Given to me over a year ago. I hook it up to my tap, making 5-6gs at a time. Get my filters/resin from BRS for $50 a year and I've always gotten 0 TDS. That's good enough for me and my tank.
Like with everything in the hobby you'll get tons of opinions. Some more adamant then others. Some experts even!
Get what your comfortable with using, what you can afford. All according to how much water your going to be using.