The C615 is a happy middleground between the C270 and the C920 (there is no C720). The C270 is great if you plan to crack it open and modify it so you can fix the focal point to whatever is ideal for your setup.
The C270 is 720P and has fixed plastic optics (that can be changed if you open it up - there are articles online and parts to print off Thingiverse to make it easy to adjust), a 60 degree field of view, and relatively basic UVC controls exposed.
The C615 is 1080P and has glass optics instead of plastic (not Zeiss optics like the C920), autofocus, a 74 degree field of view, and has more UVC controls than the C270 (but fewer than the C920).
The C920 is 1080P and has Zeiss optics, autofocus, a 78 degree field of view, and exposes a ton of UVC controls.
I speak as someone who has at least 2 of each model and is quite familiar with them for everything from basic webcam usage, though videoconferencing, and all the way up to computer vision. Links to specs are below: