I have an old sigma 70- 300 but it searches too much, I cant seem to drive it right!

Im thinking of buying a longer lens - I have the 70-200 f4 but frustrated with lack of reach! which of the above would you say was a better buy, oh I have the 2x extender but find it too slow to manual focus.
any thoughts appreciated thanks
cas

The 400mm prime will be a tad sharper, but the 100-400L is so flexible I find it pretty well indispensable as a walk around general purpose wildlife lens, whereas the prime would sometimes be just too long for the closer opportuinities which arise.
Absolutely. I certainly wouldn't want to carry one around all day, and it's not comfortably hand-holdable.Although people talk of the Sigma 120-300 it really is in a different league from a size point of view and very heavy so not really comparable.
When Canon Bob shows up he'll tell you that his 300 gets more use, but his 400 is the one with the sun tan.
IMO, if the light's right then the 400/5.6 is what you should be carrying.....if it's a dull day then the 300/4 IS and T/C is a good substitute.
Bob
Why do you recommend the 300 f/4 with TC is a better bet in poor light than the 400 f/5.6? Both are then 5.6 lenses and I've tended to find native lenses focus better than those with a TC fitted. Is it just because you can remove the TC and gain a stop at the expense of reach?
Cheers
Paul
No Paul, it's the option to use the IS for stationary tweeties.
Bob
You don't use IS on the tripod Paul?OK Bob, thanks. Hadn't thought about IS as I tend to use a tripod for static stuff...
Paul
One significant difference between them is the minimum focussing distance, which at 350cm for the prime, is almost double that of the zoom. The only time I got close to Ians kingfishers at Brandon, they were actually within the minimum focus so I didnt get the shot!!
Shouldn't that read 3.5meters or 3500cm?
350cm is only just over a foot in plain money
Is it really Chris.....that lay-by incident has had a lasting effect.
Bob
350mm is just over the foot m8
350cm is 3.5 meters