One big difference is how long it maintains 0 TDS. Things line flush kits are feel good purchases, I have yet to see a documented study they do anything other than give you a warm fuzzy feeling and lighten your wallet. Same with the bogus water saver claim. Notice not one of the long time vendors who specialize in RO sell a system line that. Truth is when you have dual membranes it is even more important you keep the waste ratio up since you are sending concentrated brine from one membrane into the next. It's not that easy, if it were don't you think the vendors that have been researching and developing RO/DI for 25-30 years or more would have been doing this decades ago?
Research the differences between nominal, near absolute and absolute rated sediment filters and why some are better than others at protecting the billion of cavities or pores in the carbon block where chlorine is adsorbed. Research why something as simple as a 2% gain in membrane efficiency can double the life of your DI resin cutting your cost of operation significantly. Find out why a treated and tested RO membrane can do this for you versus a dry untreated and untested membrane, they are not all the same. Research why a single high quality low micron carbon block is all you need and why it is actually more efficient and lower cost than unneeded multiple carbons. While on carbon find all the articles in n why so called chloramine carbons are a waste of money on EPA approved drinking water and why carbon doesn't remove chloramines in the first place. Carbon removes the chlorine portion of chloramines and breaks the bond with the ammonia portion so it can be removed mostly by the RO and polished off by the DI which is much more important than carbon.
Again there are huge differences in systems and even bigger differences in vendors knowledge and experience. Research yourself and don't fall for many of the myths and questionable claims. Stick with a vendor that specializes in RO/DI, it will save you money and give you better water quality with fewer upgrades and replacements.