Upload & Sell: On
There's got to be a reason why the 100-400 is only $360 more new. The 100-400 is a zoom, same aperture at 400mm and it also have IS. There's got to be a catch to it. They both are pretty sharp. So what's the reasoning behind buying the prime over the zoom? Is it build quality ,IQ, something else?
There are a lot of reasons for lens pricing, and the pricing isn't always directly tied to value or quality or usefulness. But rather than worrying too much about that it makes more sense to think about which if these fine lenses provides functionality more suited to what you need the lens to do.
The 400mm f/5.6 (which I do not own) is regarded as a fine lens that can provide slightly better IQ at 400mm than the zoom at 400mm. If your need is for a lens that shoots only at 400mm, there is probably no reason for you to choose the zoom over the prime. (Though there could be if space for carrying gear while traveling is an issue.) Since it does not have IS, you could find yourself a bit more limited when it comes to hand holding the camera for low light shooting. If you shoot the lens from the tripod or always shoot in excellent light the lack of IS is perhaps not a concern for you. I believe those who say that it may AF a bit faster.
The 100-400mm L zoom (which I do own) is also a very fine lens. Contrary to what you sometimes read, I get excellent image quality from this lens, even at 400mm. While I do not doubt those who say that that prime is a bit better, the zoom can do very well at 400mm, too. IS can help for handheld shooting in low light. Sometimes I'm glad I have that when photographing birds very early or late in the day, when I'm already pushing boundaries of aperture, ISO, and shutter speed. The zoom provides more flexibility, which can be very useful - when I'm shooting wildlife I can often zoom back to 100mm to photograph the larger scene (a bunch of critters rather than just one, for example) and then zoom right back to 400mm without changing lens. An additional bonus from the zoom is that it packs smaller due to the extending zoom design - which can be important when traveling.
Someone mentioned the use of 1.4x teleconverters. Both are subject to the same limitation here. Most Canon DSLRs cannot AF (without some work-arounds that most won't want to try) at apertures smaller than f/5.6... which happens to be the largest aperture that both provide at 400mm. Some Canon 1-series cameras (look them up) can AF at f/8, but unless you have one of those this issue is moot.
In the end, both are fine lenses and neither is perfect - each has its strengths and weaknesses. You can make great photographs with either, so I think it is most important to think about the functional capabilities of the lenses in context of your intended use.