This might not be a fair test but it is a good indication on how the ordinary consumer lens stack up against Pro L lens in canon range. This is just a result of what I see and tested. By no mean backed up by any high end scientific findings - just me and the elements. I will not go into particular details on how IS works or what it does - okay IS is short of Image Stabalizer for Canon lens, it can help reduce the shutter speed by up to 2 stops (if you need to know what 2 stops means - read here). Of course, IS is not everything - and cannot create background blur or stop action like a faster lens.
Simple put, say you are shooting at 200mm @ f/8 on a Full Frame camera, you would need at least 1/200 shutter speed (1.6x crop factor camera, you would need 1/300 (1/320) shutter speed). With IS, you can shoot at 2-stop lower than this, meaning you can now shoot with shutter speed of 1/50 (half of 200 is 100 = 1-stop, half of 100 is 50 = 2-stops)
EF100-400mm f/4.5-5.6L IS USM:
Big boy of the L series lens, amazing to hold, wonderful to use, frickin heavy to lag around town. Cost $1900 to $2600 AUD. My mate got it for 2G.

EF75-300mm f/4-5.6 IS USM:
A step up from the ordinary EF75-300mm f/4-5.6 III lens. It is slightly bigger and heavy than the kit 75-300mm lens. The IS does help alot. (Newer version is 70-300mm IS). I got my for $700 AUD second hand.

So, here we go.

Image below show test shots when view full image @ 100mm:

Image below shows cropped @ 100% @100mm:

Image below shows cropped @ 100% @100mm:

As you can see there isn’t that much of a different in the two images, a slight colour differences, the edges are still holding very well.

Image below show test shots when view full image @ 200mm:

Image below shows cropped @ 100% @200mm:

Image below shows cropped @ 100% @200mm:

Here there is a slight edge softness on the 75-300mm IS but the center is still sharp compare to 100-400mm L IS.

Image below show test shots when view full image @ 300mm:

Image below shows cropped @ 100% @300mm:

Image below shows cropped @ 100% @300mm:

Here, the 75-300mm IS can’t stack up to the mighty 100-400mm IS. It is very soft all over the image. Hey, if you don’t have anything to compare to the you won’t know, right? Yeah, I know the L glass still has another 100mm on top of this.

Conclusion:
They are both excellent lens, it’s all comes down to how much cash you have to spend on the glass. Personally, I don’t have the cash to burn and I went with the cheaper glass. Because at 75, 100 and 200mm, it is very sharp and on my 30D, the edge softness doesn’t really concern me that much. Different story tho if you are using Full Frame senser DSLR….it could mean useless image.