Upload & Sell: On
I'm thinking a bit about this question since I spent the day photographing migratory birds in California's Central Valley yesterday and, of course, used the 100-400 almost the entire day. Although I had a chance to review the day's work last night, I don't quite have examples ready to post, but I may a bit later today.
Regarding the sharpness of the 100-400 at 400mm... when I do post an example or two, I think that you'll have to agree that this lens is capable of producing very fine results at 400mm. In my experience it can be even a bit better at shorter focal lengths. Of course, someone will come back with the "get a good copy" mantra. There are less than perfect copies of any lens out there, but I don't really think that this is a particular issue with the 100-400.
What I think can be an issue is that shooting with a 400mm lens is not as easy as some like to think, especially if you are shooting moving subjects like I was yesterday, double especially if you have to work handheld, and triple especially when you consider the variables that can mess with focus, including atmospheric distortion and multiple possible AF targets in the frame and so forth.
Another point that may be important. As I photographed these birds, mostly in flight, rather than just getting more pictures of birds centered in and filling the frame, I think a lot about how groups of birds form themselves into interesting compositions and about how they stand against background elements like clouds or trees or the horizon. There are two points to make about this. The first one has little to do with lens quality at all - it is that technique is by far the most important thing when it comes to shooting subjects like this with long lenses. Technique includes many things that affect sharpness, but it also includes practicing enough that you can see not only the target but the whole image while panning and tracking and all the rest. Second, because the darned birds don't always fly exactly where I want them to, I often find myself actively changing the FL while shooting so that I can include them in the frame along with some of the other compositional elements - if the birds come by low I may be at 400mm, but if they are higher I may back off to 300mm so that birds and, say, a tree can share the frame. You can't do that with a prime...
The weight difference was discussed above just a bit. There is a difference, but it isn't a deal breaker. And don't forget that there is also a size difference in that the 100-400 is considerably smaller when zoomed back to 100mm for packing, so it takes less space in your bag.
Finally, if you are on the fence about whether or not you need the zoom or the prime... I recommend the zoom. The only way to answer the question of whether or not you need those other focal lengths is to shoot and find out. With the zoom you'll find out quickly whether or not you need FLs other than 400mm. I predict that you'll use them and be glad that you have them. If not, sell the lens and get the prime. The cost of doing this is low if it ends up getting you the lens that is better for your photography.