I am really happy with my RO unit. I never installed the DI. I use it for both drinking and my tanks. I like the control I have over water params. It's become one of those items I really value in this hobby, unlike a UV sterilizer (useful, but totally unnecessary).
As for measurements between RO and RO/DI, RO/DI should read very near zero when testing TDS. My RO only, with a taste filter that raises TDS slightly, reads around 80 TDS, which is still really good. About the only thing left in the water are the charged ions, which I don't think really hurt anything. I have tested KH, GH, nitrates, and pH, and KH/GH/nitrates are zero, while the pH is a perfect 7.0. While TDS is a pretty worthless measurement inside the aquarium, it is useful when testing RO water.
Having one for a 20g tank is overkill, but if you would like your own 'endless' supply of Dasani, I would recommend it. And definitely if you plan on using RO water with multiple or larger tanks.