The main thing to keep in mind is that this is a situation where more is really less.
A 75 gallon per day membrane removes more nasties than a 100 gallon per day unit.
If you know your tapwater TDS and it's real high, sometimes it's cheaper in the long run to have two separate DI resin filters. In this way, as opposed to having a mixed bed resin filter which have to be thrown out when exhausted, you can go to separate cation and anion color changing resins. These resins can be "recharged" with normal household chemicals so you only have to buy them once.