Field sports: Canon 300 2.8 IS vs Sigma 120-300 OS Sport

[iGotter]

Looking to spend some extra cash for a field sports lens for the fall. Unfortunately the 400 2.8 is outside of my price range. Not looking to spend anything more than 4k at this point.

Right now I am shooting with:
7D
17-55 2.8 IS
70-200 2.8 IS II
85 1.8

At some point, I will probably be adding a second body, most likely a 5D Mark III since that will help me out quite a bit for basketball come winter time. But I don't think I can swing spending the money for the lens AND the body right now.

So for now, I am just looking to pick up a field sports lens for use with my 7D.

I am conflicted between the legendary Canon 300 2.8 IS and the newly released Sigma 120-300 2.8 OS Sport.

Since I will most likely only have one body for this fall, I'm leaning more towards the Sigma for it's zoom range, but I haven't seen too many sample sports images from that lens or even heard much about it from sports shooters. The slower AF from the Sigma worries me a little bit. IQ I'm also not too sure on. Plus it weighs significantly more as well.

I'm not necessarily looking for a concrete answer, but I would love if some who are more experienced with these lenses would offer their thoughts about this decision.

 

[2n10]

I do know it can be used for birds in flight quite well so it may not be that much of a problem for sports use.

 

[macroman1]

Hi iGotta

Check this photostream - some awesome shots of motorsports, military aircraft in flight etc. Almost exclusively 120-300 (pre-Sport, from the dates), several with 1.4x and even 2x.
http://www.flickr.com/photos/damoblack/with/9604600100/

He's obviously very good at handling this big beast!

Cheers