I always recommend the 100gpd over a slower unit. I know others recommend a 75gpd unit because many have a better rejection rate. Here's the thing: If the water coming out of your unit is 0 TDS at 75gpd or 100gpd, does it matter? What you want is pure water, and personally I want it as fast as I can make it.

The unit I use for my own tanks is 100gpd. That means I can make 5g in an hour in the summer. In the winter when the water is colder, it takes 85 minutes to make the same amount of water. If you have a 35gpd unit and water conditions are optimum, it'll take 1.5 hrs to produce a gallon of water. In an emergency, and those do happen, you need water and you need it now.