As far as RO goes...
Most all of them start out the same, most use the same canisters, and parts. the only differences are in the filters themselves.
Membranes are critical, I use 3M filmtec membranes.
The prefilters are important too.
For DI units that use resins, some resins are better than others also, but all will become exhausted and eventually need to be replaced.
You could spend big money on an RO unit now, and replace the components with cheaper ones and end up with a unit that is not as good as it was new, or you can buy a cheap unit and put good filters in it, and have something better than it was new.
Hopefully that makes sense..
I knew what I meant when I started typing.