I have owned both of the the Canon 100-400s and also the 70-300L.




* 70-300L About $1500 used, can't find one new.
http://tannin.net.au/browse.php?firs...rd=1&second=14
PRO: A very sweet little lens, surprising how much smaller and lighter it is than the 100-400s. A joy to use. Very sharp, handles well.
CON: Too short! Not much use at all for birding, of limited value for wildlife, and although the 70mm end is sometimes handy, I still prefer having the significant extra reach of a 100-400. Good IS and AF but not quite as good as the 100-400 Mark II. In particular, lacks IS Mode 3, which turbo-charges your ability to shoot sharp pictures in poor light.






* 100-400L Mark II.
http://tannin.net.au/browse.php?firs...rd=1&second=18 About $3500 new, $2200 used.
PRO: Arguably the best EF telephoto zoom Canon has ever made (or any other company for that matter). Handles well, top-class Mode 3 IS, fast focus, optically superior to any other lens in its class. (Only the big white primes beat it, and they are $15k and heavy as lead.) Although notionally shorter than the third party "600mm" zooms (the Sigmas for example), its optics are superior and it delivers a sharper picture even after cropping to equivalent fields of view. Plus its smaller, lighter, and has vastly superior handling. Best in class, daylight second.
CON: price, weight (which really isn't a big problem).







* 100-400L Mark I.
http://tannin.net.au/browse.php?firs...rd=1&second=17 Might be $750 used but I'm guessing.
PRO: quality optics at a fantastic price. A very old design now but still a better than just decent performer. Like all these lenses (and especially the other 100-400) offers you the ability to reach out into the landscape and pluck exactly what you want from it. It's a great photographic rule: decide what it is you like about the scene, show that and leave everything else out. Rugged and reliable (well, so are all the other L Series lenses).
CON: the least optically impressive of the three (but still more than good enough for almost any task); very old but perfectly functional IS system; you don't get bragging rights (do we care?).




I like third-party lenses. I own and happily use a Tamron 85/1.8 and an ancient but still delightful Tokina 10-17 fish. In the telephoto zoom arena, however, none of them can hold a candle to the Canon lenses. You can get cheaper and inferior; not much cheaper, slightly inferior, and much, much heavier and clumsier; or much cheaper and barely worth having. In particular, the various "something-600mm" zooms are heavy, clumsy, and inferior in almost all respects. They look great on paper but in the real world the 100-400 II creams them every time.

Your best choice? The 100-400 II is THE best lens in this class at any price or with any brand. The 100-400 Mark I, if you can find one, will do pretty much everything any of the others do, and do it very nearly as well, and cost you three fifths of five eighths. And the 70-300 is a real sweetie. Among these three there is no wrong answer.