Buy a unit with a 10" verticle DI chamber -- it'll be more efficient and is much easier to refill with bulk resin. Also, to really get the best rejection rates, stick with a unit that has a RO membrane rated for no more than 75gpd (unless it's running 2 75gpd membranes together to conserve water).
Technically, any RO/DI unit should produce 0 TDS water if the DI resin is good and there aren't problems with channeling through the resin (a more common problem with horizontal DI cartridges than with vertical ones). The real point to make, if you're interested in long term efficiency, is that the better the RO membrane, the less the DI resin has to work for you to end up with 0 TDS water. The less the DI resin has to work, the longer it lasts. The longer it lasts, the less money you spend since you're not buying DI resin as often.
