By far, the best answer is both, but if you HAD to chose - An RO would be the best/only decision. A quality, properly configured RO can remove up ~98% of the TDS in most water with the DI resin removing the remainder. It would not be feasible to use just DI in most scenarios as it would not provide the mechanical and prefiltration of the RO and would exhaust itself almost instantly becoming cost-prohibitive.

We work so hard to limit/remove nutrients and contaminants from our tanks, I shudder at the thought of knowingly introducing them with every gallon of fresh salt or top-off water.