The problem with an undersink unit is where are you going to put 20-30 gallon of water that you'll need for water changes? The under sink units are also designed for small volumes and make water at a slow rate. I started off with a 50 gpd unit but quickly upgraded it to 100 gpd because it made water so slowly.
The main advantage of a RO/DI is lower TDS. An RO will get you down to about 10-20 TDS. An RO/DI with fresh cartridges will get you down to 0 TDS.
Gary
125 SPS, 75 gal. LPS/softie reef, 9 gal. Nano