RO/DI water and Membranes

The membrane I use in my RO/DI system is a Filmtec by dow. I am sure Walter could write a book on this subject but I am interested in the different brands out there and why some are better then the other.
Just what makes them better.
I have a 100gpd Typhoon II and was told that a 75 gpd is better at reducing TDS.
Why is this?
I always check my reading with the TDS meter I have and they are always very low. I mainly get 0 reading and change out everything but the membrane when seeing 3 to 4 TDS reading.
How accurate is the TDS meter and is it really a big concern that they are - + 1 ?

My tap water reads 180 to 250 TDS.