EF 70-200 f/4 L IS +extender vs. EF 100-400 f4.5-5.6L IS

As this is my first post, I apologize in advance if I missed a previous post on this exact subject or a big flashing link that said "Glenn - this will answer your question!".

I've owned a Rebel XT for a while and have been using the 18-55 lens that came with it, but I've missed having a zoom lens. So I'm trying to figure out what I want to buy, and I came down to two options that covered the range I want.

Even though I'm not a professional by any stretch of the imagination, I narrowed things down to the following:

EF 100-400 f4.5-5.6L IS

or

EF 70-200 f/4 L IS +extender (1.4)

I'm kind of torn because I can get either of these options for about the same price (I'm not quibbling over that). I guess I'd like some general opinions, as I might have missed something.

I think the advantage of the 100-400 is that has the extra range, but it's heavier and bigger. The advantages I see with the 70-200 are it's lighter and has better IS, but the range is decreased (even with the extender. I'm not sure how much the weight will matter to me - I'm young and in good condition, but I'll be carrying it around (especially in travel).

Any advice would be greatly appreciated!

Thanks!

I would be interested in hearing pros and cons on this arguement as well.

The last two years I have shot airshows with 100-400. This year I am shooting with a 70-200 f4L IS with a 1.4x TC. Hoping to get even better pics out of the 70-200 setup. Some of the shots with the 100-400 came out a little (ok, a lot) soft. I am so far attributing that to the fact that the lens was a rental.