Inline TDS meters are not as accurate nor can they be used portable so you are stuck with two monitoring points. A good handheld is worth its weight in gold.
Filters Direct is Water General on ebay and thats exactly what you get, a mediocre ebay quality system suited for drinking water and not a true reef quality system with low micron filters, a tested RO membrane and a 20 oz vertical DI filte rfilled with fresh reef specific DI resin. You really do get what you pay for with RO/DI and cheap is not where you want to be.
I don't work for any RO vendor or manufacturer but I have owned many units over the last 20 years and am a water treatment professional with 37 years experience so have more than a little experience with all of the components and manufacturers on the market. I have personally helped many many other reefers completely rebuild Filters Direct units so they do what they are supposed to which should not have to happen. There is no comparison, just look at each components description yourself and make up your own mind.
Even though AZ and I differ on the most superior unit, he knows his sh** and his feedback is always quality...