I have both and run them at the same time. I found that the 230 doesn't squeegee and scrub properly because the robot is so light the brushes sometimes do not fully contact the floor. You can even see the bristles just skimming the surface if you get down and look. solution? I velcro a 6" x 1/4" steel disk on top of it and it then performs as intended. It picks up double the liquid with the added weight. Doesn't seem to tax the motors either. They really should have made this robot heavier or made the squeegee hang lower and brushes longer.
I wish the 230 would spend less time room crossing and wall following and more time spiralling and shimmying, by which I find it cleans best. It spends so much time room crossing that the solution just dries up before it returns to that spot.
The 390 doesn't get in between the toilet and sink, and other tight spots the 230 can. The 390 also cleans much better than the 230. Every time I run it, the water is black. The water never gets very dirty in the 230, so I know it's not cleaning as effectively.
When I run the two machines together in large areas, I use water only in the 230 so it just helps by going around and rinsing and does a bit of extra scrubbing in the corners and other areas the big one can't get at.
The 230 only cleans small areas because the thing doesn't put enough solution down to do long room crossing, it just leaves a light streak of wetness that dries up,instantly. However, It really shines in the bathroom and small foyer areas. The 230 is NOT worth $300 and you will feel much remorse long after you bought it. The 390 is a great machine, highly recommended.
Current bots: iRobot Roomba 880 & 770, 2 Scooba 230s & 2 Scooba 390s, Braava 320, Braava Jet 240, Robomow RM510 lawn mowing robot
Past bots: iRobot Dirt Dog (wheel seized), Roomba 560 (fried circuit board) Roomba 630 (found new home)