720p is technically "lesser resolution" but it is still a high definition (HD) class of camera. 1080 offers better resolution if all other factors are equal (i.e. lens quality, lighting, recipient screen, streaming quality...and an app that can support 1080). My experience is that when in a standard video call, lighting/focus/microphone are more important than 720 v. 1080.
I have not compared the C505 and the C270 directly, however, the key feature with the C505 is the "long range omni-directional microphone." I can attest to clarity when more than 1-3 feet from the camera. I tend to pace in my office when on a video call. The microphone picked me up very well no matter were I stood--even if I was behind the webcam. You also get 2 extra feet of USB cable with the c505.