ASU2003

Thanks for the advice so far. I'll explain a little further now that I have some time.

I have used my little point & shoot as a camera until now. And it has 1x-10x zoom (28-280mm equivalent they say) And I would say 10% of my pictures are at this 10x range. For 5% of the pictures, I would like to get zoomed in even more, and 5% are in the 5x zoom - 9x zoom range. It is those pictures in that closer range that worry me about not having the zoom lens.

What I am taking pictures of... I am planning to keep this lens for 10-20+ years, so a wide range of things. The things I have taken pictures of before are sports from the bleachers, football (day and under lights, and I can't always move around to get in the right location), baseball, beach volleyball, hockey, bike races, Indy 500, olympics, golf, space shuttle launches, zoo animals, animals in the wild, birds around the lake, air shows, and possibly macro. The main issue is that I'm not sure 300mm will be enough for some of these events, 420mm (with 1.4x) would take a few seconds to put on or off. 70% of the time it won't be an issue, but there are plenty of times where I want to frame something that is moving fast. If an airplane is flying towards you, an indy car is going around a corner and your seat means that the corner is too close to get the full car in the shot, a bike rider is going from 500m to 3m away from you (I may use a close up lens in that case and lose the far shot). And then again, it might be my lack of photographic technique or vision that is the issue.

The thing is, I am looking for the sharpest pictures at the pixel level. I have to take lots of pictures to get ones that come out right where the fast moving object is sharp and in focus (and framed correctly).

And I am planning to get a full frame camera, so it's not going to be quite as zoomed in as if I had a smaller cropped sensor. I might be able to borrow a 500mm lens to do some test shots to see if I like having one focal length. Though I risk getting spoiled after seeing what it can do, and I can't justify buying a $5000+ lens now, I barely can justify buying a $2500 camera & $2500 for two lens.

This article is why I am having problem picking a lens.
Forgotten 400
The prime lens at 400mm in that review could be digitally enlarged to the equivalent of 600mm or 800mm before the blurriness equaled that on the 100-400mm zoom (at 400mm) at 100% (*I'm not sure if the test on that website was flawed, using a unfocused lens, focused someplace else, a 'bad' copy of the 100-400, or if it is a real result.). I run into this problem with my small camera, where there is so much noise that anything over 50% zoom looks blurry and noisy. My thoughts are that the 300mm at f/4 would be better in lower/worse lighting conditions, but would also have sharp pixels that I could digitally enlarge/crop a picture to the 400-600mm range if I couldn't get close enough. I am using a 21MP sensor for a reason . That article said that using the 300mm with the 1.4 extender would reduce the quality (probably to around the 100-400 level), but I would like to take or see a few test pictures for myself to really know if it would impact things a lot.

It appears that other people have had this dilemma before. It also doesn't help that you never know what Canon is going to come out with in the next few months. So, if they came out with a 400mm f/4 for $1500, well I would have to re-evaluate everything again.