Hi Andy
The both make pure water but they differ in the way they do it.
DI is a container of resin that fits on to a tap and makes pure water, by filltering through the resin beads. This is fine if you have a low tds reading at the tap, tds = total desolved solids.
Ro makes pure water by pushing water through pr fillters then through a fine membrain taking out up to 98% of tds. then it goes through the resin. The resin in this system lasts a lot longer as most of the tds has been taken out before.
First thing to find out is what is your tds reading at the tap.
Are you on a water meter?
Regards Roy