If you're not going to pixel peep, why even buy a 6D at all? You're talking about going from an older Rebel to full frame, yet you're worried about sharpness, but you're not going to pixel peep? Doesn't really make sense to me. Spending that much money and not being serious enough about the files you produce, doesn't make sense. To each their own, though. You really don't need a $2k body if you don't pixel peep.
I disagree with RS. To duplicate the reach of a Sigma 18-125 lens on your 550D, you would need a 28-200 lens for a 6D. So that would be a center crop resolution equal to about 180mm on the 6D. If you take a lot of pictures of animals, the 24-105 on a 6D, will not be enough of a telephoto. 105mm on a 6D, is like a 60mm lens on your 550D. Do you shoot many animals at 60mm? If you do, and you rarely go above that, then by all means, the 24-105 will suit you.
This is a problem for people getting into full frame. You assume you can have a compact lens for walking around like you did on the smaller camera, that will give enough reach for wildlife or animals...but it just isn't going to happen. Basically, you will have to give up the wide end, and get a 70-200...I suggest an f/4, without IS. $600...cheaper than the 24-105, and as sharp if not sharper. I've rented the 24-105, and have owned the 70-200 for 3 years. You could also just buy both lenses.
If you're interested in taking good pictures of animals, especially anything that moves relatively fast, you might as well spring for a 5D3. The 6D is best suited for slower action, and for portraits.