I have a 50D and the 100-400, so I can offer some insight.
First, I had to micro adjust mine +6, and that made a huge difference right off the bat. Still, it is soft wide open at the 400 range. But, if you put it at f/7.1-9 it is sharp as a mother sticker. This means you need light - there is no way around it. When I am using it I immediately set my f stop to 8 and my ISO to 400. Then I see where my shutter is and go from there.
On the 50D ISO 400 looks just as good as 100, I'm usually able to get a fast enough shot with those settings and they look absolutely great.
In terms of dust, I have none. I bought mine used from lensrentals.com so I don't know about it's previous use, but you can bet it took some intense situations before it got to me. I've take mine to the beach during a frickin wind storm and have had no problems.
I don't get why people think that push pull means dust vacuum. Any lens that moves forward and backward will push and pull air. The sealing is what makes the difference.
Anyway, if you have a bunch of cash and want to wait a year or so then sure, wait. No doubt the new one will be nicer. But for $1,300 you can get the current one used and I promise you will love that thing. You can always sell it and get the new one when it comes out. You might lose a couple hundred, but that's much cheaper than renting it for a year. You might also realize that you don't want the new one because the current one is a great lens already.