How large of a TV does it take for you to start noticing a difference between 720 and 1080? [BluRay]

My main camera records 1080i 29.976fps and my high speed records 720p 60.00fps, so I'm not sure which way to go? If I up-size the 720p footage to 1080 it might look bad when crunched back down on a small TV? Or would I even notice much of a difference if I just downsize my 1080i to 720?

Aside from BluRay I assume all HD Youtube video (not in full-screen) looks best in 720p rather than 1080p