The 75 or Spectrapures 90 GPD will produce treated water faster so less waiting around.
If your system is automated and goes in to an ATO storage container and turns on and off by itself its no big deal but if you are using a faucet adapter and making water manually then the larger membrane might be a blessing as it frees up your faucet faster. It takes hours to make much usable water and I know my wife would be upset if I had the kitchen faucet occupied for 8-10 hours straight making reef water. I ended up installing my system over the laundry sink in the garage and using a brass wye off the washing machine cold water supply so nothing is affected when its in use.

The Dow Filmtec 25, 50 and 75 all are 96-98% rejection rate, the Dow 100 GPD iso nly 90% rejection so not suited for our purposes since it eats DI resin like crazy. Spectrapure takes that same 75 GPD and treats it with a special proprietary process to improve that rejection rate to better than 98% so DI lasts longer and the GPD goes up slightly too. For every 2% you increase the rejection rate you double the life of your DI resin so a new membrane can pay for itself quickly if yours is less than 96-98% efficient.