400mm 2.8IS w/1.4 vs 600mm 4.0IS

Does anyone have any experience with these 2 combinations? I know the 400mm w1.4 ext will be 560mm 4.0. I am interested in image quality.Also is IS less effective with an extender? I am leaning toward the the 400mm just for the low light speed but also want the focal length of 600mm. Most of the time I will be using the MarkIIN.
Thanks Bruce

If I wanted "reach" I'd be using the 20D myself. From everything I hear the 400 does very well with TC's. But, if you are doing small birds and such wildlife, you will never have a long enough lens. Then get the 600, it also does well with TC's. Do you have a specific need for long and fast (I'm thinking sports here,) then get the 400.

I've shot with the 400f2.8IS on a 1DS (without TC) and it's quite amazing, obviously. But the standard rule applies: go with the focal length you need most. If you will be using the TC on the 400mm a lot, go with the 600mm. If you shoot mostly around 400mm yet need the occasional longer reach, then get the 400mm. Hard to find 2 better lenses.