I bought my RO/DI setup (a Typhoon III) at airwaterice.com (AWI) and have been happy with it.
Bob has a point, a lot of 100 gpd membranes ahve a stabilized rejection ratio that is less than many of the 75 and 50 gpd membranes (90% vs 98%). That means that less is removed by the membrane and is left to the DI resin to remove. That, in turn, means the DI resin will not last as long before needing to be replaced or recharged.
However, there is a 150 gpd membrane available with as good a rejection ratio as the smaller membranes (98% efficiency), so the argument is not without exceptions. (See here:
the complete FilmTec line of nano and reverse osmosis membranes
Check out the FAQ at AWI if you want to learn more about RO/DI systems in general.
BTW, I've also heard good things about The Filter Guys....