I have the T2i (550D) and T3i (600D) and they are basically the same. The T4i, T5i, and T6i seem to be only minor improvements in ISO, saturation, and video/still quality as well.
Now that the full frame 5D classic is so cheap. How does it compare in low light with the Txi APS-C cameras? I've been having trouble finding comparison crop shots to see the noise levels in lower light (indoor and outdoor). What I have found seems to show sharper images with lower noise.
Note: I resize almost all my photos down to 1080p or 2k for online use so I don't need much more than the 12MP the 5D classic provides.