Scooba 230 vs 390
images from manufacturer

Cleaning Stages:
The 230 only includes a 3 stage cleaning system, you have to sweep the floor before using it. Whereas the 390 has 4 stage cleaning, so no sweeping required.
The Scooba 230 3 stage cleaning system: (1) Wash, (2) Scrub, (3) Squeegee vacuums up.
The Scooba 390 4-stage Cleaning system: (1) Vacuums, (2) Applies cleaning solution, (3) Scrubs, (4) Squeegee vacuums up.

Floor Coverage:
The 390 covers large areas up to 425 square ft. per cycle.
The 230 covers small areas up to 150 square ft. per cycle.

Approved Cleaning Solutions:
The Scooba 390 works with Scooba Hard Floor Cleaner, Natural Enzyme Formula, water or water and vinegar
The Scooba 230 works with Scooba Hard Floor Cleaner, Natural Enzyme Formula, regular tap water

Price: The Scooba 230 is much cheaper.
Scooba 390: $499.99 at irobot.com
Scooba 230: $279.99 at irobot.com, price of the 230 with essentials kit is $299.99.

Size: The Scooba 390 is larger.
Dimensions of the 390: 14.8 inches in diameter, 3.6 inches in height
Dimensions of the 230: 6.5 inches in diameter, 3.5 inches in height

Weight: The Scooba 390 is much heavier.
Scoomba 390 weighs 8.6 lbs.
The 230 weighs 3 pounds.

Buy the Scooba 390 if you need a more advanced robot that can clean a larger areas.

Do you know more difference between Scooba 390 and 230? Please share below.