Upload & Sell: On
I'm coming late to the conversation but hey, better late than never. I'd say the major consideration in your prime vs zoom decision is whether the 100-400 would just end up zoomed to 400 90% of the time. If so, I'd give very serious consideration to the prime. But if you'll really be using the 100-400 throughout its range, than by all means, go for it! I own a 300 f/4 IS, 500 f/4 IS and 100-400 (and 70-200 f/4 IS). A few years ago, I began to become addicted to whale photography and at that time only had the 300 (a very fine lens!) to use (the 500 being far too heavy). For my style of shooting, I found the fixed focal length too constraining with whales. I decided to try a used 100-400, expecting to actually dislike it. Well, I've gotta say, the IQ on mine is just fine and I love having the ability to frame my shots within its zoom range (I use it primarily on my 7D but also on my 5D2). Granted, the IS is rather ancient and inefficient and it may not AF at lightning speed and I find that it's sort of fragile (as have others - don't knock it against anything), but despite my initial misgivings, I'm about to put my 300 up for sale since I just never seem to use it anymore. Now, if I found I was using the 100-400 at 300-400 all the time, I wouldn't be selling my 300 but I'd be thinking about buying a 400 f/5.6 and selling the 100-400
Good luck with your decision!
PS: I find dust a non-issue, have ended up liking the push pull zoom, against all expectations (feels quick and natural when dealing with quick changes in focal length) and altho, as has been pointed out, the AF isn't wildly fast, in my experience, it's at least fast enough to use for breaching whales if nuthin' else.