Need a techy person to really explain but in my simple terms its this.
RO - Using filters and membranes the tap water is purified to a minimal TDS reading and then finally polished of using a DI vessel.
The benefits are the water is produced at the cheapest cost.
Downside is unless you have a static tank your vehicle is stood overnight filling up. Ours takes 12 hours.
DI - A cannister that basically you send tap water through that then gets reduced down to 000 TDS
The benefits - Quick fill time, mobile in van
Downside - COST !!!! The more you use it the quicker it will be expired. I used a 12.5 ltr DI vessel and was averaging 1000 litres at 000 before the levels started to rise. Bag of 25kg Resin is about £60 - £80
Hope this helps Mark.
Best wishes,
Trev