actually.. have been considering picking up both.

100-400 f/4.5-5.6L IS USM $1400

<img src="http://www.bhphotovideo.com/images/largeimages/162616.jpg">

300 f/4L IS USM $1100

<img src="http://www.bhphotovideo.com/images/largeimages/129188.jpg">

I would really like the 400 f2.8L IS but the $6,000 sticker is a bit too much for our budget. Yes even the 400 f4L IS DO is a bit much at $5,000.

<img src="http://www.bhphotovideo.com/images/largeimages/186152.jpg">

The majority of the photography we do is motorsports... I found the 70-200 f2.8L to be great with the 2x but want more range... also the 70-200 makes for a fantastic portrait lense too..

any thoughts? suggestions? thanks!

we're not pros... some people expressed interest in using our services so we started one up. We have 3 guys in the crew and it would be nice if each of us had a lense with some range.. hence the idea of picking up both. Plus... the 300 is a prime lense...there's something to be said of a prime lense over a zoom lense in sharpness and quality.. no?

Works pretty good, the push-pull takes a bit to get use to but it's pretty intuitive.

It just seemed a bit redundant for one person to get both.

100-400 - haven't used it, but read quite a bit about it. Zoom range is ideal for motorsports, and it's got IS (bonus). It's reported to be reasonably sharp to 300mm, and then sharpness falls off above that. I'd love to have this after shooting at the USGP carrying a 70-200 (which never got used) and a 300 f4 IS.

300 f4 IS - a smidgeon sharper than a 70-200 f2.8 with a 1.4x. But when used for motorsports, it's unlikely you'll notice the difference. Technique will play a bigger role here. I do like the 300 for it's lighter weight though. I can handhold that all day long, where fatigue eventually sets in with the zoom. Another alternative to the 300 f4 IS is the older non-IS version. I've seen comparisons, and the earlier non-IS is definitely sharper. It's up to you whether you're willing to make the trade off. As far as focal length goes...300 wasn't enough for me on a 1D. I forgot my 1.4x at home, but I think that would have fit the bill nicely.

You might be really happy with the prime, however there is something to be said for the flexibility of the zoom...and the zoom also precludes any need for lens swaps in the field (tracks aren't the best places to be swapping lenses). The only downside is a lot of people call this lens a giant dust pump. It will likely draw dust in when you're zooming. And if I had to guess which one will make you more money...I'm betting it's the zoom!

I haven't tried anything as fast as motorsports yet, so I can't comment on that. I love the 400mm reach. I've never shot with anything over 300mm before this lens and I have to say that extra 100mm makes a big difference in a stadium atmosphere when you may not be so close to the action. This isn't the fastest lens in the world though. f4 at 100mm isn't very fast at all, but plenty serviceable in daylight and well-lit night time stadiums. f5.6 at 400mm is actually really good for a zoom lens. I don't think you'll find a faster 400mm lens for the price, certainly not a faster zoom.

Here are some sample photo albums with this lens. I just bought it, so I am still getting used to it.

<a href="http://homepage.mac.com/jctrac/PhotoAlbum17.html">Fenway Park at night (MBWW, these are full-res pics)</a>
<a href="http://homepage.mac.com/jctrac/PhotoAlbum23.html">Wrigley Field, day game</a>

You can tweak the manual focus ring as you are pushing and pulling. Sometimes, it isn't so easy to keep a fast moving subject within one of the cameras AF points. If you're a little bit off, the camera will tend to "search" for an AF point. Those few milliseconds often make the difference between a nice shot and an out of focus blade of grass. I like the ability to zoom in or out while tweaking the FTM focus ring at the same time.