Hi Paudie.
An RO/DI unit will always be better than just an RO unit.
Some contaminants do not measure well with a hobbyist grade TDS meter, it takes a conductivity or resistivity meter to measure them. Some of these include phosphates, silicates and nitrates among others. Which means that a TDS meter can show 0 TDS, even if phosphates, silicates and nitrates are present. RO membranes don't deal with this sufficiently. But the DI resin filter will.
So by combining a DI resin filter to the RO membrane, you get the purest water.
I would suggest a DI filter instead of another carbon cartridge, if you can fit it in.
Note: when we talk about 'RO' water we usually mean water that has been filtered through an RO/DI filter.
DI filter cartridges expire very quickly. And is usually the first cartridges that needs replacement when you start to see TDS rising from your RO/DI unit.