The push me pull me is a dust hog. Birding is field photography ergo I find the 400 prime a better selection.
The "dust hog" or "dust pump" nonsense is just that - a silly internet myth propagated by people who have never actually used the lens.
If the OP is planning to buy a newer lens, he shouldn't worry about getting a "soft copy" - the early problems with quality control were cleaned up about 10 or 12 years ago (this lens has been in production since 1998).
Why would people lie? I heard that the 100-400mm and the 17-55 were dust collectors. I don't have any experience with them, but I do stay away from them because of the rumors. Of course I have spent a ton of cash on other lenses... so it's not like I'm missing out.
So you might have missed out on using great lenses because of those rumors. The rumors did their job!
I had the 17-55 for about five years, and it gathered a few specks of dust that I could see, but nothing that would qualify as a "dust pump." None of the lenses are air tight, so dust can get in any lens. If you have questions about a particular lens, it's more accurate to get informed opinions from those that have used the item. The internet has many opinions and a lot of them are wrong or not representative of the collective user base.
I wasn't interested in the 17-55 because I knew at some point I was going full frame... so that was NEVER on my radar screen. As for the 100-400... I just don't like the pump... it's wholly irrational, but it just doesn't appeal to me at all.
I quite honestly didn't like the slide action of the 100-400mm either. I bought it because that was the only way it came and I needed the flexibility out in the field to go back to 100mm or 200mm without changing lenses. The 100-400mm served that purpose. A little MFA on both my bodies and it's been a wonderful lens and a joy to use. I got used to the sliding telephoto action quite easily and I've never regretted the purchase, although I had a lot of doubts when I ordered it, due to the sliding barrel, before using it, it just didn't set well with me.
I would be an adopter of the next generation for sure. That being said, although I am well acustom to the slide, I'd much prefer the next version to not slide...