[sassan]

Harry consider today as your lucky day as I happened to pass through the same path you are getting through so may be my experience would be useful for you.

First I think your options are not balance in terms of money as you first package is about 60-75 % higher than the second option. Well you may as well get what you are paying for.

70-200mm L IS by many quotes is one of the best lenses Canon ever made. Is the highest priced in it family of four; F/4, F/2.8 non IS and new F/4 with IS (BTW the F/4 in absolute terms has better resolution that all but you can find the subtle difference only on film with no distinctions what so ever on digital media). Many call it a Tank, at least its weight does feel that way. However image quality is like nothing you have ever experienced before. Believe me when I say that as I am one of those disbeliever that won't get convinced easy especially when I am to pay top price for something. Image color render and capability to deal with direct light falling straight into lens without any internal reflection is a unique delicacy of this baby. This feature is brought in by not only the patent unique Canon's coating on glass but by especial way in which surface of lens is cut, that is not a smooth surface but micro roughness and indentations that cause that especial damping of internal reflections. From the day one having it I know its just like my wife, "Love for ever". It has replaced a lot of my prior lenses and is always with me even on far far journeys despite the weight. The only cons I can quot is heavy weight (I consider that as a way to strengthen my neck and shoulder muscles). And the second nag is for probably my excess expectations as I was thinking I will not see the blur image any more. Still that happens, so no matter how good the IS is still don't forget your measures to be steady.

2x extender is the other lens that I have and use commonly with the big baby. This is the extender most photographer don't like in favor of 1.4x . I am not so sure. I have sharp eyes for Chromatic aberration and other glass additive problems, I see none with this one. Things you want to remember;
- AF won't work if your lens has F narrower than 2.8 (With 1.4x the limit is F/4).
- It won't fit on lenses 135mm or shorter in focal length.
- There are look alike old and newer versions, Mk I and Mk II. You don't want to pay for the older version, no matter how cheaper it may be as the difference between them is not one of face lift but major architecture changes and MK II has a weather sealing, newer coating in addition to optical improvements. May be one reason many don't like this one is most tests are from the older version.
Cons here;
- You will see some decrease in AF speed (Not too bad though)
- You loss 2 stops
As user, I love it and use it commonly. Sometimes can't which image was with extender, unless looking at EXIF.

For your second choice 100-400 that is a very popular lens between bird and animal shooters, picture quality is still excellent though you don't have that fancy F/2.8 that can make significant difference when light is low or bird is a fast flying one. With adequate light you possibly won't see much difference and have the convenience of not going through the add/remove process of extender. I don't have this lens so can't help you any further.

I have the Bigma 50-500mm and if one wish, would like to see Canon making L class in that range. The lens was my favorite far reaching lens before 70-200 IS. Though still I like this lens a lot but image quality especially in terms of color saturation and render, is not a match. Bigma is a lot lengthier in highest focal length. Narrow F stop was the other hassle and needless to say when use this lens with sigma 2x, AF is dead completely and out of focus images a common problem as the view finder doesn't have split prism or any manual focus assist. Of course now we are talking of 200 or I believe 150 to 1000mm range. ( When extender added to 50-500, due to bulge of 2x elements into original lens barrel, you need to move the focal lens of mother lens to 100 so losing the some of the low focal lengths.
My Bigma may soon find its way to ebay sadly.

Conclusion; If money not a hassle go for your right instinct i.e. 70-200 with 2x extender. Don't forget you are choosing better good and better so no matter what you chose, I am sure you will be happy with the result. I am hearing a lot of good about the 28-70 F/2.8 Bigma so no matter what, you may want to keep it unless you are planning for more wallet damage and may want to consider 17-40mm L or 10-22mm EFs or may be both. I have them both and usually the 10-22mm is the other lens always traveling with me, though 17-40 is one of the very popular ones by professionals and incidental the cheapest L class Canon, may be one of the first steps into L for many photographers to own and become believer of "L" class quality

Hope this help and treat use with nice images once back home safly.