Membranes have only x amount of gallons they can produce before they start to foul. Most membranes are the same when it comes to that, whether it's 75 or 100 etc. Say they are all rated for 3000 gallons, with the 100 you can hit is sooner if you use it, that doesn't mean it's better or worse but it works better for your application.
What I'm talking about is that in some membranes the 100 actually doesn't produce quite the same quality as a smaller one. It allows more through to hit the higher gpd. The more product or waste in x amount of time means less quality product. Lots of membranes are say 96% in 75 gpd and a little lower at 100. It basically just uses up your DI more. I just figure why not go for a little better rejection rate and save money at the same time. Now if you need the higher gpd then go fo it, it will be plenty good.