Most are all the same. Sediment filter, chlorine block filter, membrane, and a Di resin chamber.  
Some have extra filters or resin chambers, double membranes, and built in Tds meters. 
Imo, get a 75 gpd unit as thats the most common size for replacement membranes. 
There is only a couple of membrane manufacturers, and they just get re-labeled for use. 
Check out prices from the vendors here before u buy. 
If it doesn't have a built in Tds, them make sure to get a handheld one as that is a necessity with an RO/Di. 
TB aquatic is a vendor here and has great prices on replacement filter packages. 
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