i think you wanted to compare the 651 with the 620, the 651 has an obstacle detector and a virtual wall. i've had the 620 for a year or so and i'm completely satisfied, but an obstacle detector would be nice as it sometimes (especially with black objects) bumps into things really hard. i don't know how well this detector helps, but there are certainly tests on youtube.
other than that, the robos are identical in the way they are used. what amazes me, however, is the low weight of the 651, which at 3.6kg is considerably less than the 620 at 5.5kg.... However, I don't know which is better in terms of suction technology.