300/4L + 1.4TC or 100-400mm?

I am looking to improve my capabilities in the 200-400 range. I already have a Sigma 50-500mm which is one of the best Sigma lenses out there, but it's not "L" glass.
I also have a 70-200/4L lens which I like. My initial thoughts were to go for the 100-400mmL lens to cover my needs, but having read several mixed reports on the "push/pull" zoom function and the benefits of prime lens over zoom, I am not sure. I wouldn't be at 400mm all the time. I do like the idea of the IS function having struggled with the Sigma lens without a tripod! I have read very good reports on the 300mm/4L lens itself and with the use of the 1.4X extender. Does anyone have any comparisons between the 100-400mm and the 300/4 with and without the extender? Does the extender degrade the "L" glass images? I can see a benefit to having an extender for my other lenses too, but I don't want that to sacrifice my "L" performance.

Tough call. You already have two great telephoto zoom lenses. I have the 300 f/4LIS and the Canon 1.4x teleconverter. I am very happy with both. I haven't done any scientific tests to determine if the teleconverter degrades the prime lens. I'm sure the laws of physics say yes, but the pictures look as sharp and color correct to me.
Does your 70-200 f/4L vastly outshine your Sigma 50-500 EX? If it does, then maybe you should go for the 100-400L IS. But don't lay down a wad of money because of peer pressure to collect L glass. I still eye your Sigma zoom from time to time and the 100-400IS too. I bet the owners of that lens wonder if my prime IS lens is sharper.
It's all good.
Not meaning to insult your intelligence, but for the benefit of some that will read this thread: do you have a monopod. I just bought one, and love it. I use it a lot of times that I wouldn't use a tripod. Talk about adding f stops to your telephoto lenses. I would wager you get better results with your Sigma at 500 on a monopod than I would with my 300IS/1.4 tele hand held.

But, if you do buy one of these lenses, be sure to give us a comparison review.


p.s. short answer to your question: 300/4L + 1.4TC

Which lens is recommended by professional photographers if they have to carry only one lens, not just a Canon, but among all lenses? A sports photographer, I think, recommended Canon 100-400mm L. I also recommend it, if you are using it outdoors. Not indoors, because at the end of the zoom, the lens will not collect much light and IS can't help you. It is also the heaviest among zoom lenses.

The extender is good, but it is a headache because you have to attach it before the lens. I bought one (1.6 or something) several years ago, and used it only once during a graduation ceremony.

If you can move about a little, get the prime. If you have to take pictures stuck in a corner and mobility is limited, then choose the zoom. You will be happy with every picture from the prime, but you cannot always get one. You can always get some pictures with the zoom, but will be happy with some of the pictures when you use the middle of the zoom. I never had an occasion in which even getting a low quality picture was important.