I posted something longer in the other similar thread...
But largely comes down to...
Best image quality - the 6D.
A pro-quality body - the 5Dii.
Remember, the 5dii is 4 years older, which is an eternity in technology. In terms of image quality and features, the 5dii probably has few, if any, advantages over the 6d. But it has a body meant to withstand shooting in a hurricane.
If you only currently have the t1i and kit lens, realize that when talking full frame, body is only the beginning. Lenses tend to get very expensive. And it's a waste to buy full frame if you aren't going to eventually be putting some pretty good lenses on it. Figure you may eventually spend 2-5 times as much on lenses, as on the body.
But also, if you only currently have the t1i and kit lens, then the "whole world is your oyster." Canon cameras are great, no question. But so are Sony, Nikon, Fuji, Samsung, etc. There is little reason to stick to a brand, if you aren't carrying over an expensive lens investment. "Brand familiarity" is pretty irrelevant, the learning curve doesn't change dramatically when shifting brands. (The bigger part of the learning curve will be going from an old entry level camera to a current enthusiast/pro grade camera). The 6D may be the best low light traditional dSLR for the money... But a camera like the Nikon D750 isn't necessarily that much more, and carries a lot of advantages including a far more advanced AF system and higher image quality.... A camera like the Sony A7 and A7ii is much smaller, with the same level of image quality, and a whole different set of pros and cons (with the A7ii, in body stabilization, more accurate AF thanks to on-sensor detection, big 100% viewfinder, but slower AF, smaller lens selection, etc).
A pro-grade camera is like a marriage --- because once you start buying lenses and flashes for the camera.. easily spending $1,500 to $5,000 to $10,000, you are pretty committed to the "system." (I know the difficulty.... I "divorced" Sony A-mount after many years, to re-marry Nikon). You're currently in the situation of being totally "single," so you have the chance to really pick which system you want to marry. The Canon choices are fantastic, but so are the other brands. So you have a chance to really decide what best suits you.
& I have other lenses I purchased the 18-200 mm (I think that's right) so I didn't need to switch from 18-55 & 55-200 (both came with my camera as a black Friday deal) and the 50mm (the cheap one). I'm just swaying on staying crop or going FF.
