70-300mm L image cropped vs 100-400L?

Hi all...
Been digging around on POTN but can't seem to find an answer...

I have both a Canon 5D and 5D2 bodies in nice shape, and currently have the 100-400mm L lens in good shape. Together they produce some good images, but at age 68 that darn white pipe is getting heavy, so am looking at replacing it with the newer 70-300mm Canon L lens.

Assuming identical f stop and pretty much same light conditions, how well does a 300mm cropped image on the 70-300mm L lens compare to a 400mm image on the 100-400mm L lens using either of the 5D and 5D2 bodies?

If weight is the issue, I would just like to point out that the 70-300L is not much lighter that the 100-400L... 1050g vs. 1380g.

A 24% decrease in weight is pretty significant.

I have the 70-300 non L and the 100-400L. The IQ of the 70-300 is very good, the lens is light weight and is a reasonably good outdoor all around lens. But if I want to shoot wildlife it is some times a little short. Then again sometimes my 100-400 is a little short. It just depends on which of your needs has priority.

Mike

I just pulled the trigger and ordered the 70-300 from B&H, should be here tomorrow and ready for my trip to the Alps in Switzerland, plus the 16-35 L II Ultra wide angle.