Of those models, I would take the 720 (its the highest end with the best camera, display - along with the 620, and best battery life.)
In a nutshell:
620 - most compact, high quality display (480x800 - 3.7" - sharpest, yet smallest), smallest, yet replaceable battery, extra features over the 520
625 - raw size, the largest display, but not the sharpest. About on par with the 520 in panel quality I believe. (480x800 - 4.7" - least sharp, but biggest), non-replaceable battery, but has largest pack, on par with 720. 2000 mAh) has 4G LTE connectivity unlike the rest
720 - middle ground - high quality display, (480x800 - 4.3"), best camera, same non-swappable battery size as 625. wireless charging option. horsepower is about equal to the 520, 620/625 are a bit faster I believe.
630 - I would put it in between the 520 and 620 in most regards. (480x854 @ 4.5") Large battery. (1850~ mAh IIRC)
Theres more minute differences, but I think that's the jist of it.
But the midrange is due for an upgrade eventually, so I would wait. even if for some reason the 30's don't do it for you, the 20's will be cheaper..