A 100gpd (or higher) RO membrane removes ~90% of the TDS from the incoming water. A 75gpd (or less) RO membrane removes 95 - 98% of the TDS from the incoming water. RO is a waste intensive purification process. For every 1g of good water coming out of a RO membrane, there is typically 4g of waste water. If operating conditions are less than ideal, that ratio of 4:1 can easily go up to 5:1 or even 6:1 (temperature and pressure are important factors).
Using only DI, there is no waste, as the water is pushed through the DI resin and out the other end. However, for most people, it is much more expensive to use only DI due to their incoming water quality, how quickly DI resin is used up, and the relatively high cost of constantly replacing the DI resin (compared to the relatively low cost of water).
That said, the best water purification method for the bulk of us is a RO/DI unit. The RO membrane removes most of the impurities, flushing them down the drain, while the DI follows up on the RO product water by removing any remaining ions, ultimately producing water with 0 TDS.
In looking to buy one or more units to handle that much water per week, Freddybnj, I would highly recommend testing the TDS of your incoming water before spending the money on any unit. Once you get a rough idea of what the incoming TDS is, you can better decide which type of unit (RO, DI, or RO/DI) will work best for you. Although, any 75gpd RO membrane (be it on just a RO unit or on a RO/DI unit) would be able to make enough water for a weekly water change in < 2 days. If you had the system automated with float switches in your storage container(s), it wouldn't require much attention at all while making water for you.