SkipD wrote:
Having a kit of lenses with these radical differences is like trying to drive radically different cars at random times. You never get comfortable enough to have your actions be truly "learned" and automatic when the chips are down and you need to make an instantaneous decision. You have to constantly think your way through the controls when all your lenses (or automobile controls) aren't the same. This will bite you in the butt at times.
That is very true for some and not for others ... same with many comments when these choice options are discussed. Glad you mentioned 'some folks' 
Radically different ... lenses? Personally, I have had Tamron SP, Tokina Pro, Sigma EX, Canon, Canon L, Zeiss, Pentax, and Jupiter over the last year or so and I never once remember turning rings the wrong way, other than perhaps the first time or two.
Some may have the opposite experience. That's life. I suspect it's like the car mechanic who jumps in and out of 15 cars a day and never stalls once, while someone else takes time to adjust and maybe stalls a few times because they are so programmed to drive the same way. Preference more that big deal. That's fine.
As to resale, I've mentioned so many times it's getting boring to me and no doubt everyone else. I depends. Buy junk and you get next to nothing. Buy well reviewed, popular lenses of any brand and you will do well on depreciation. The two lenses I made money on SH where the 50-500 Sigma and 70-200 2.8 Sigma. I have yet to lose much on any lenses, but I shop around, buy low and/or SH. Buy a Canon L at full retail and sell it 6 months later then I suspect you'll lose more than I've lost on all my lenses put together. I guess if I totalled all my buys and sells I'd be around 50% better off on the 3rd party stuff depreciation compared to the L's.
I think JoJo sums it up well. There are many better reasons to want the Canon rather than the competition at this range. Work out what you need and then buy what you need. Or, if you prefer, just buy what you want. I could afford any of them if I really wanted to. I've owned the Tamron and the Sigma and had the 24-70 L for an afternoon to see what the fuss, one way or another, was about. I bought the Sigma. No IQ differences across the three for me and the Canon was big, heavy and clumsy. My opinion and I'm happy others think different. No problem.
Had I wanted to use silent USM in the rain I'd have bought the Canon. Had I wanted something lighter and smaller and longer for travel I'd have kept the Tamron. I wanted fast enough AF, decent build quality, IQ and some more on the wide end. It did the job. The only zoom around this range I'd buy other than the Sigma right now would be the 24-105mm IS L.
Horses for courses. Personal preference. No right answers. Most of the 'stuff' people seem to find so important in their 'gear' is actually of little important in the overall scheme of things and what is actually needed to take a shot worth looking at: imagination, exposure and composition.
What I find using primes with 100% manual operation and focus is that my keeper ratio is radically up and my shots are better and need little PP. Bigger, faster, costlier isn't always better ... at least in my experience. Often slower and considered gets the results. Hare and Tortoise