Platinum it all depends on your TDS reading of your tap water, if its 050 or less then I would say DI, but for most of us our tap water reading is anywhere between 180 and 550, so in those instances you will need an RO system to bring the reading down to about 010-030 then even with an RO it still goes through a DI vessel to finish off the filteration process.
runnin just a DI setup is quicker than RO but is more expensive, 25kg will set you back £60-70 and wouldn't last you that long at all, probably 4-5 weeks tops, thats with a tap TDS reading of 300.
The advantage for the DI is if you run out of water you can top up anywhere.
With the RO you have to either go back home to your static tank and fill up or connect a water source to the filter in your van and wait hours to fill your tank up.
Hope this helps.
Matt