The difference between Roomba 650 and 620 are as follows.
Roomba 620 vs 650
This is the key difference between them. The Roomba 650 has scheduling feature that allows you to set it to run automatically at a certain time for up to 7 times per week. This is convenient if you hate hearing the nosie. You can schedule it to vacuum when you are not at home.
While the 620 does not have this scheduler.
The 650 comes with a Virtual Wall that keeps Roomba cleaning the areas where you want it to clean and stops it from entering off-limits areas.
While the 620 does not comes with the Virtual Wall.
The 650 is in darker color while the 620 is lighter.
Otherwise, they are very similar.
They employs AeroVac cleaning system, Dirt Direct Series 1 to sense dirt, have a recessed one-hand carrying handle, etc.
And both of them are NOT compatible with the Room to Room navigation kit – Lighthouse, do NOT have full bin indicator.