Newbie needs help choosing lens

I am new to dslr photography and need help choosing a lens for a Rebel XT. What I need to know is the difference between the 70-300mm IS USM (black lens for about $650) and the 100-400mm IS USM (white lens for about $1550). I understand that the zoom is greater on the 100-400 but is that the only difference? Is the picture quality difference worth the extra $900?

The only reason I am considering the more expensive lens is because I have seen pictures taken with this camera lens combo. The subject matter was the exact same thing I will be using it for and the pics looked wonderful. If the picture quality is the same between the two lenses, I think I can get buy with the 70-300. I don't want to spend more than I have to, but I don't want to buy something that won't do what I want it to.

Just to let you know, I will be photographing remote control helicopters that are flying. I want to get blade-stop, or close to blade-stop photos. The photos I saw with the Rebel XT/ 100-400 combo were great. Could I except the same performance with the 70-300 if I were standing closer when taking the pictures?

Thanks,
Medflight

Although there's an image quality difference, you're paying the big bucks to get an image-stabilized lens up to 400mm at f/5.6. You need to ask yourself whether you need 400mm or not.

I think I could get by with 300mm. It's the image quality that I'm most worried about.

Image quality is alot better on 100-400 L lens, the difference is the 70-300 USM is a consumer level lens and the 100-400 L is a pro unit. The 100-400 L may also be slightly faster on the focusing but I can't say as I've never tried the 70-300 USM IS. Anyway, it's not just the extra 100mm thats the only difference, it's the quality of the optic's and also the build is better too normally.

The 100-400L is certainly a handy lens and a quite good one at that.You will find many happy owners of this lens here. ( I went with a different lens myself but I have tried it).
I think for most people the only issue they have with it is getting used to the push/pull. I don't know if the EF70-300 IS USM is good enough for you (only you and your budget can decide that) but IMO the 100-400L is a better choice for the price than the other for it's asking price.

Go for the 100-400 it is a superb lens.

Would I get better pictures with a smaller zoom lens and a 1.4x TC? Since most of my pics will end up being cropped anyway, could this be something that would work? Maybe not even using a converter?

You could, but if you are buying the lens and the converter only to get that extra reach, you'd be better off going longer. However, of course if its a budget issue, sure, but that 70-300 already is slow so adding a extender would make it very slow.

If you can afford it, you'd be better off with the 100-400. There's no way that 70-300 will stand up. Canon has done a really poor job with their 70-300/75-300 series lens.

Sigma has a 100-300 f/4 for around 600-700 dollars, which would probably be better. And it's not as expensive as the 100-400.

Do I need IS and USM? I've got it in my head that I do.That's why I was looking at canon lenses.

Well the 100-400 has IS on it. So it makes it more than capable to handhold. Many people on this forum love this lens. And for a zoom, its a very capable one. You'll only really get better results with a prime(fixed lens of like 400mm).

Thanks for the help. Guess it depends on what I want to spend. I can afford the 100-400 but don't know if I want to drop that much on something to take pictures of rc helicopter. May get a good quality,cheaper lens to start with for generl shooting (something like the canon 28-135 is usm) and look for a good, used 100-400.

Sigma has a 100-300 f/4 for about 700. That might be better suited for you.

If I had the choice between the two and could afford it, I'd get the 100-400. L glass produces such wonderful colors, and the clarity is second to none (except for a prime, of course).

I'll check out the sigmas if you don't think I'll need IS and USM. Thanks.

Well, you might be able to handhold, you might not. Depends how steady you are. That's where IS comes into place, but if you think the cost is too high, that's a great substitute.

The 100-400 is an absolutely great lens. And IS is very useful in marginal conditions (which start around 1/500 sec. at 300 mm). If you want to get rotors that are blurred a little, you're going to need to either be pretty close with a shortish focal length or use IS or a 'pod to get a slow enough shutter speed without camera shake. "Frozen" rotors and props don't really convey the same impression of action as that bit of blur does.