RO or not?

If you're going to be using just the DI, keep in mind: The TDS of whatever the feed water is going into the DI will determine the lifespan of the DI. Check the TDS of your feed water and divide it by the capacity of the DI to get an estimate of your DI's lifespan.

 

The water in my area is 500 tds coming from the faucet. If I ran my water through a di system, 2 of the standard 10 filter housings, I would exhaust them in less than one week. I do recharge my own resin, but do not like doing it that much.
I also run a 7 stage system that started as a cheap Ebay unit. My finished water is at 0 and I recharge when it moves to 2 tds. This happens at 6 months very regularly.
Some folks run stand alone DI units but this is not possible where I live. Knowing what your faucet water quality is important when deciding DI or RO/DI.