The 650 is so much lighter than my 505, as if it were empty inside.
But side by side, I didn't really see that much of an advantage in the screens over the 505. The 650 definitely has a sharper screen, but overall they were very close in readability. At least under the lamps at the Sony store. Couldn't move the 650 to less ideal lighting. But I lost the urge to upgrade at the store...
As far as page turns, the 650 responded much quicker to the actions, but the actual page turns were about equal too. So if I pressed next page on the 650 a few moments after the 505, they changed at about the same speed.