Below are some difference between Scooba 390 and 5800:
Scooba 5800 vs 390
images from manufacturer

The Scooba 5800 is an early model of the Scooba floor washing robot, introduced on 2006, whereas the Scooba 390 is launched on 2012, it is the lastest Scooba. They are safe to use on sealed hardwood, linoleum, marble, and tile floors. But not recommended for use on unfinished, unsealed hard floor surfaces, or laminate flooring.

* Area Coverage
The Scooba 5800 can cover an area of up to 250 square ft (1-2 rooms) on a full charge, whereas the Scooba 390 can clean up to 850 square ft (1-4 rooms) per battery charge, 1 full tank washes up to 425 square feet before it requires refilling.

* Efficiency
The Scooba 390 moves faster than the 5800.

* Noise Level
The Scooba 5800 is louder.

* Color
The 5800 is blue while the 390 is iron grey, and the parts of the Scooba 390 that require cleaning or maintaining are now in grey and orange color.

* Battery Life
The Scooba 390 has longer battery life for the benefit of the extended life power management.

Please share your reviews if you know more difference between them.