Thanks for all the advice, if anything I will at least be making a very informed decision! The 24-105L is best for me. I can easily stop it down to f/5.6 at 24mm to improve sharpness and CA. Not a big deal. That's the only real advantage for the 24-70/4 has that I can tell from looking at TDPs lens compare tool - better at 24mm f/4. At 24mm I'm likely taking pictures of a building or a landscape in which case I'll be stopping down anyway. At the 70mm end both lenses seem to perform about the same in my opinion. And at the long end is where I might use f/4 for a portrait. The 24-70/4 does seem overpriced now. User reviews remain mixed. Some love the new lens and some are quite dissapointed. I wonder if the ones who love it are just using it at the extremes?
I'm thinking I might even hang on to my 17-55 for now and run both lenses for a while and see how I get on.
Cheers and have a great weekend!
I will say none of the three 24-105 I tried had good edges or corners at 24mm even at f/8 or even f/10. From what I saw the 24-105 seemed to handle charts better than real life scenes with complex DOF, it depends what you shoot though. Only the 24-70 II and 24 1.4 II got that. My Tamron 28-75 2.8 had sharper corners and edges at 28mm f/8 than the 24-105L too. But the 24-105 can be had for a low price these days. I suppose you can worry about Tamron QC, but then you can worry about it for many lenses. My first copy Tamron 17-50 and 28-75 were good. My first 17-40L was bad and 24-70 II had . My Canon 70-200 f/4 IS was good. So it's luck of the draw all around I'd say.
True about the lens lottery thing. My 85 1.8 had way too much CA even up to f/4 yet people raved about that lens. As someone who likes backlit shots it was pretty useless. And I had a Canon 10-22 that was a bit soft though some say it is sharp and better than a 17-40L, I disagree. Even at 17mm it performs quite well and it's a great lens for the price. If I ever need ultimate sharpness in a WA I'll opt for a TS. I'm not earning much from photography to justify it yet.
I'm sure the Tamron is a great lens, I just don't like the way it zooms and I heard AF Servo is hit and miss. Plus they're a bit harder to sell. I'll take my chances with the Canon. And in any case I'm sure it'll be much better than the 17-55.
For the 85 1.8 I assume you mean Longitudinal CA where it gives that purple friging (or green behind the plane of focus)? Yeah that lens is very much known to have a lot of LoCA. I'm a bit surprised it was so extreme even at f/4, I mostly shot it f/1.8-f/2.8 when I had it. Yeah white sports uniforms under indoor lighting was enough to get purple showing all over, never mind back lit type stuff. It was relatively free of lateral CA though and mad sharp.
17-55 is basically an L so don't expect the 24-105L to be some miracle compared to it (I think it is actually a bit less sharp comparing both on same aps-c body) and on FF it gives fuzzier edges, even f/8, than the 17-55 does on aps-c (talking wider end of each). But maybe it'll be good enough for you. The price is excellent for it now and it's not much risk to try.