This should put things in perspective, taken from another simular thread...
Originally Posted by amfoto1
I voted "other"... by which I mean both are equally viable options and neither is perfect. There really is no such thing as "best"... It's what's "best" for you, what meets your particular needs well, and what fits into your kit of gear best that really counts.
The Tokina 12-24 is a nice lens at a reasonable price. It's well made, is fairly resistant to flare (a common problem with UWA... just the nature of the beast with a lens that has a wide angle of view) and has quite good image quality. An f4 lens is plenty fast for an UWA most of the time, too ... So long as 12mm is wide enough for you. There are 10mm and even 8mm wide lenses available. (Note: it is wide enough for me... I have and use a 12-24 and don't have plans to exchange it for anything anytime soon.)
The 24-105mm is a fine lens, too, as might be expected with an L-series. Very well made, some sealing, USM for fast/accurate focus, reasonably compact. Very nice IQ. But, relatively pricey and some vignetting at the wide end when used wide open (won't show up much on a crop camera). I prefer to have f2.8 on a middle zoom, so use the 24-70L instead... Though I have a 28-135 IS as a backup/walkaround lens, too, that cost me $250 used. There isn't a lot of difference in image quality between the the 24-70, 24-105 and 28-135 IS. The 28-135 pretty much matches the 24-105 at focal lengths they share... at 135mm the less expensive lens is a little soft wide open. And it's a variable aperture (f3.5-5.6) lens.
I like the combo of 12-24 with 28-135 (or 24-70/2.8 and 70-200/2.8). These are good ranges of focal lengths for me. Actually, 12-24 combined with 28-135 on a crop camera is complete coverage of a wider range of focal lengths than most film shooters ever owned in their entire lifetimes. (This pair of lenses is roughly equivalent to a full frame/film kit with 20mm to 215mm. Many film shooters bought a camera with approx. a 50mm lens. Then the most common additional lenses they would buy were 28mm, 135mm and/or 200mm or 80-200/70-210. Pretty few film shooters ever bought wider or longer lenses than those.) Brand new 12-24 and 28-135 together cost less than the 24-105 alone and about the same as the 17-55 alone. (Both 12-24 and 28-135 also can be found used for considerable savings, making them even more of a bargain.)
The combo of 11-16 with 17-55 comes up short, with 55mm at the longest. That might be fine for some folks. Others might happily use this combo along with a 70-200.
The 11-16 is the only f2.8 UWA lens. It has good IQ and great build, but is more prone to flare than its cousin or some other UWAs. It also costs more than the 12-24. And the trade-off to get f2.8 is an very narrow range of focal lengths. Most zooms are 2X at least (12-24, 16-35, 17-40 for example). This one isn't. Do you really need f2.8 on an UWA? It's not going to blur down backrgounds very much. In fact with wide lenses most of us tend to stop down most of the time to be able to get greater depth of field.
Yes, surely there are some people who absolutely need the speed of f2.8 for low light shooting. The next closest lens is the new Sigma 10-20/3.5 that's 2/3 stop slower. OTOH, an UWA lens is pretty easily handheld at relatively low shutter speeds, perhaps offsetting the need for f2.8 to some degree. Certainly the high ISO capabilities of newer cameras also offset the need for f2.8 to some degree, too.
And there are alternatives. Of all the UWA lenses, the Canon 10-22 is the best for controlling flare. It's unusually good in that respect and other aspects of its image quality are about equal to the Toki 12-24. It's a little more platicky build than the Toki, but it's impossible to say if that translates into less durability. The Canon is a USM lens, while the Toki isn't (not that USM isn't as important on an UWA... the focusing elements only need to move a very short distance to do their job, so even without USM focus is near instantaneous and the inherently deep depth of field pretty much hides any minor focus error). There's also a noticeable difference between 10mm and 12mm wide. If I were looking at the 17-55 as my mid-range zoom, I also might consider pairing it with the new Sigma 8-16mm instead of the Toki, just to have that additional width. Seems a neat lens, though the unavoidable wide angle distortion effects get pretty strong at 8mm!
The EF-S 17-55/2.8 is a fine lens optically, too. Folks love the images it makes. It's nice to have f2.8 on this lens, in the middle range and potentially using it for a short portrait lens around the 55mm setting. And it has USM. But it's fairly expenensive, some have trouble with dust getting under the front element and there have been reports of the IS failing in the past. (I've seen far fewer reports of either of these two concerns recently, maybe Canon has quietly made some changes to the lens.) And 55mm would come up short for me... Though it would pair very nicely with a 70-200 (I wouldn't worry about the gap between 55mm and 70mm). It is an EF-S lens, which sort of rules it out for me since I shoot with both crop and FF cameras and prefer lenses that can be used on both (the Toki 12-24 actually can be used on FF as wide as 19mm approx... though I have a 20mm lens that I normally use instead because it's well corrected, has less WA distortion effects).
So, really, I think you can pick either pair and be satisfied with it... Whichever appeals more to you or fits into your kit better. Several of the lenses mentioned have IS, which is always nice, though for me it wouldn't be a big priority on lenses shorter than 100mm.
At the wide end, even a millimeter or two can make a noticeable difference. So I'd try to minimize gaps between lenses there. Some overlap such as you'd have pairing up Canon 10-22 and 17-55 really isn't a problem... It can mean fewer lens changes in the field. At the tele end, a modest gap between focal lengths isn't an issue... such as 55mm to 70mm, as mentioned above. Overlap, such as would happen with 24-105 and 70-200, isn't an issue, either... unless bordering on fanatically trying to put together a minimalist lens kit covering as much range as possible. Someone wanting a 100-400 or 120-400 or similar telezoom might look at the 24-105 (or 28-135) instead... though the gap between - say - 55mm and 100mm isn't necessarily a problem either.
