400 2.8 or 500 f4 for BIF

I'm definitely getting addicted to BIF photography just having taken it up over the past year. I currently have a 7D and a 400 5.6L. A buddy that I shoot with just bought a 500 F4 to go along with his 50D. Very nice setup.
I was wondering, other than the weight factor, is there any reason not to buy the slightly cheaper 400 2.8 IS over the 500 f4 IS. I'm talking used version 1 lenses not the new Mark II's. I've been watching the prices of used ones and it seems like the 400's are selling for less than a comparable 500 as of late.
I would more than likely have the 1.4 extender on the 400 90% of the time giving me 560mm. Is the reduction in autofocus speed while using the 1.4 extender on the 400 even a concern or is it not noticeable?
I definitely want a little better low light capability over my 400 5.6.
Any opinions would be greatly appreciated.
Thank you.

I don't have any experience with the 400mm f/2.8 but I do own the 500mm f/4L along with the 400mm f/5.6L. I like the extra reach the 500mm offers and I also use the 1.4III extender on my 500mm. I do not see a noticeable (sp) difference in focus speed using the extender on my 500mm lens or image quality ( I shoot with a Mark IV) and my main persuit is BIF's.

The 400mm f/2.8L is a great lens in it's own right....I guess it boils down to $$$$ and how much reach you need/want in your situation. (In my situation I sometimes wish I had the 800mm f/5.6L and probably why I use the extender most of the time).

The weight difference isn't a factor to be lighty discounted.

I sometimes hand hold the 500mm for fairly long stretches. I could never do that with the 400 f/2.8

Ordinarily, I'd tell someone to take the extra stop over the extra 100mm of reach. In ths case, take the 100mm and run!

Id say the the weight factor has to be considered as well.I've rented the 500 f/4 but never the 400 2.8.
After dragging the 500 f/4 around for a few days my thoughts weren't "I want a heavier lens."
You can do an great deal with your 400 5.6 and good light and I know you said you wanted lower light capabilities.
If heavy isin't an issue for you why not consider a 600 f/4 ? None of them are inexpensive.

400/2.8 is going to get you a little less each which might actually be a good thing if you are photographing BIF at good locations (i.e. where you can get reasonably close). The extra stop of light also gives you the little extra in shutter speed that will help you get sharp images.

However, the 500/4 is great in most other areas. It is lighter to carry around (some even hand hold it for shorter time), gives you a little extra reach with good AF speed.And it still takes the TC relatively well.

So basically, if you are specialising in BIF only in decent locations, go for the 400/2.8 and be prepared to carry the weight. If you want something a little more flexible, go for the 500/4.

I think most people opt for the second option, which might be why they are selling slightly higher in the used market. When they were sold in stores, I believe the 400/2.8 was more expensive.

For both, include a really good and sturdy tripod in the budget. You'll need it.

My main lens is a 400 f2.8 I love the lens for general wildlife work, but for birds in flight, I could not recommend it. The weight is very significant and will cause you to miss shots that you may have gotten with the 500 f4, which is significantly lighter and easier to swing into position quickly.

Longer is always better. Honestly I have been using a monopod with my 400 5.6 and with a good tilt head on it it is very easy to work with.

Thanks for the input. I haven't ruled out any of the super teles. the 400 2.8, 500 f4 or the 600 f4. I wish I knew someone local who had the 400 2.8 just so I could check out the weight issue.
Another variable to throw into the mix. Is image stabilization a must have in your opinion? The friend of mine who just bought the 500 f4 says he seems to be getting more keepers of BIF with the "IS" turned off. He's only had the lens a couple of weeks so I'm sure he's still learning with it as well.

No Angle wrote in post #14164158
Longer is always better. Honestly I have been using a monopod with my 400 5.6 and with a good tilt head on it it is very easy to work with.

You're using a monopod now huh? I love mine. It's almost always on my lens.

As for IS, I personally don't think its needed for BIF.
For BIF you need a fast SS so IS isn't as necessary and you don't want the IS to blur your pic while it is activating.

My best BIF shots are with the IS off.
YMMV

I'm not using a monopod now although I did buy one. Doing all handheld as of now.