I currently own a Canon T3i, which I use for movie making. While I am happy with the video quality. The temptation to upgrade is always looming. Would a 6D be a worthwhile upgrade? If not, what would be another good option. Money is a factor, 1500 would probably be a max. I could probably also sell the T3i for a few hundred.
Skip that go Panasonic gh3. Video is far superior on it by far. If you can upgrade from the rebel then do it.
The 6D is good and better than the t3i but so is the gh3 and you can get that used pretty cheap and have goooood video with it.
I have a t3i too and I've rented the 6d. Love it. As soon as I have the extra coin I'm bumping up to the 6d. Full-frame at cropped pricing. Comparable to the 5d, but the 5d is still better I think. For video, it's great and it will use all your canon lenses (check with canon first so you don't hit the mirror or something).