Upload & Sell: On
Daan -- I recall from our discussion on the 16-35 II that you are quite discerning and critical of optical performance (not unreasonably so, just very clear about lens quality), and you have very high standards. I'm going to try to interpret what I've read and seen about these two lenses based on that.
I've been spending a lot of time reading and analyzing the photos provided in tests.
One of the best on the new Canon was by Roger of Lens Rentals, showing with the tear-down how much better engineered the new lens is compared to the old -- I was very impressed with Canon's new design.
The general tone of all reviewers is that the new Canon is a big improvement over the old in all areas, with the exception of bokeh (OOF discs) and wide open edge sharpness, both attributes being slightly better in the first version. Otherwise -- center sharpness is much better and the edges equalize in quality at mid-apertures.
The reviews on the Tamron are more problematic. The IS is a big plus for many reviewers, even more important than optical quality. The images I've seen have very poor corner performance at all apertures to f/8, and edges are acceptable, but not great. Center sharpness is terrific. I believe Roger also did a review of the lens, and I was far from impressed to see the front element literally glued into position -- I expect better engineering and consider the front element the "blast shield" of a lens and want it very stable and well aligned. The general tone of reviews for still and video were that if you could live with excellent center sharpness and softer edges and OOF corners, this is a great lens. The lens seems most useful for video shooting (See Dave Dugdale on YouTube). If IS is a priority, this lens is the only game in town. But for edge-to-edge technical rendering, this is not the way to go.
The price of the Tamron is most attractive compared to the new Canon. For less than the price of the Tamron, you can have a nice used copy of the original Canon version. But if IS is critical need, the Tamron will look very good for that feature and its price.
Personally, being careful where my limited upgrade funds are spent, I don't see enough improvement in the new "L" to warrant upgrading from my excellent version 1 model. I'm not saying the new one isn't better, but I'm not unhappy with the lens I have. I'm more in need of a top imaging body like the 5D3 or 1Ds3 than I am of an very expensive lens. A good version 1 is still a very useful lens, but it does suffer in comparison to less bulk and weight of version II, even if optical were the same or close. That savings weight and size might be worthwhile to some shooters.
I don't have any interest in the Tamron, other than appreciating their effort and that it might spur Canon or others to include IS as a standard feature in lenses.
Anyway, those are the views I've taken away from reading review and watching videos.