Looking for thoughts on a new lens purchase

Back in November, I purchased my first DSLR, a Canon T3i. Now I'm getting close to purchasing what will be my primary spotting lens. After much thought and consideration of my options, I seem to have settled on either a new Sigma 120-400 OS ($900 ballpark) or a used Canon 100-400L ($1200 ballpark). While the Canon is the gold standard, the Sigma tends to get very high marks. This seems to be especially true for sharpness as multiple reviews that I have read have the Sigma meeting or exceeding the sharpness of the Canon at all except the longest focal lengths.

For about a week, I had my mind made up that I was going for the Sigma However as I kept reading some questions came up. Of particular interest were questions regarding the after-sales experience with Sigma as well as future compatibility with Canon bodies. I also am slightly concerned with the full-telephoto sharpness of the Sigma. I have always had an interest in shooting at long focal lengths, not just for aviation.

I have a few parameters that I have imposed upon my search. First, I intend to keep whichever lens I purchase for quite some time, and I do not plan on upgrading it for several years. Second, I have every intention of the photos that are taken being used in some sort of public forum. While I haven't settled on exactly which form(s) that will take, it will be more than just posting here and to social media sites. My wife's only requirement for this purchase, considering the size, is that I do something with the photos other than collect them on my hard drive. Finally, I only plan to make my final decision after I have physically tried both contenders out. While I haven't yet done this, I feel like I am at the point where I need the thoughts of others so that I can get a good background of knowledge before making my final decision.

So, great NYCA hive-mind, what do you think?

Canon. You shouldn't even hesitate. The ONLY reason I switched to Canon (from Olympus) was for that lens. You want that sharpness at the long focal lengths, and the 100-400 will be guaranteed to work with later Canon bodies (the Sigma may not since they don't always design for backwards compatibility).

Now, I will admit I've used lenses that are sharper than the Canon, so if sharpness is your holy grail, maybe the Sigma is the way to go. However, how often do you plan to print posters that are 4 feet across? If you'll never do it, then the sharpness differences will basically not matter.

TIS

Well, I *may* be selling my 100-400, so if you decide to go that route, send me a message :)