I am still in the process of setting up my 90 gal. Filling it up is quite the task. I saved up and went straight for an RO/DI unit instead of buying water. I can tell you this. It is quite the process filling jugs, buckets, rubbermade containers, whatever, just to get enough water to start a tank (can't forget the 40 gal sump, gotta fill that too!). But after its running I know a RO/DI will save trips to stores.
That's been my experience as a saltwater novice just getting my feet wet. In the end I think it beats hauling buckets of heavy water in and out of your car.