"Upgrade" from "L" to Sigma 120-300?

I'm thinking the 120-300 2.8 would be idea for LL baseball, and other sports. I just don't know if I would want to sell my 100-300mm f/4 to get it. While I use a monopod, I can handhold the f/4. And I don't know if I would like giving up 20mm on the wide end.

But I would not buy it for the reasons the OP gave.

I dont understand why not keep the 400 and use a TC with it instead of a shorter lens, if reach is what you are after?

The lens he has doesn't have IS either.

As to some of the other comments above that the larger sigma zoom can't hang with the Canon 300 f2.8, not so. The L is better, but oh so marginally. Where I have notice a difference is in build quality. My Siggy is about two years old with very regular use in commercial sports work. It is absolutely showing signs of wear. The coating is starting to come off, and I have to tighten up the lens about once a month. The weight of a 1D hanging off of make the mount come loose over time. Very easy to fix, but a hassle none-the-less. I would say my 500 L is showing some of the same signs of wear on it's coating, but it has remained very tight.

I am a little surprise at Cecil's comments that Canon lenses are a fore-ever investment. This is far from the case. Ask anyone with a mk I 400 f2.8 like I did, or one of the 200 f1.8 if Canon will service those. Not a chance. Yes they hold their value... but honestly any good lens does. I have seen used 120-300 Sigma selling used for more then I bought mine for new.

Now back to the OP's plight. If your primary goal is to use the lens with a TC all the time, you might find yourself disappointed. Unless your shooting with a 1 series body, focus reliability is going to go to hell. I think the body used with has a huge impact on the users experience with the lens. On my 20D in the dark, the siggy is a sluggish and will hunt. Same conditions on the 1d bodies.... it's a very happy camper. I shoot in fact a lot of indoor sports with it without issue. The 20/30d just doesn't have a sensitive enough AF system to play nicely in those conditions.

I would recommend looking for a used big Siggy or Cannon. You should be able to find a used Siggy 500 f4.5 out there for under 3K. The Canon can be had for a bit more. Good luck with your quest.

I had the same idea as fritz1 but he beat me to it. You should keep your 400 since you love it. Might as well get an extender on you 400 instead of getting a new Sigma.