I see that you're considering 620. I have one and love it. Plus, it is quite unbreakable. I dropped it to the floor from hip height for about three times (with two of which having the shell detached from the phone). It still run fine and no observable dent on its surface. I think a detachable shell acts like a shock absorber..
I sometimes envy my wife's 720 bigger screen. And its 6MP camera (vs 620 5MP) seems to capture more details, at least to my eyes. But it's not as unbreakable as 620. She dropped it once, and there is now a very annoying dent on one edge of the shell. I guess the shell is very thin. Consider buying the cover if you pick 720.
In any case, I'm sure you will love any Nokia WP8.