canon 300mm 2.8 or 400mm 2.8 which is sharper (1 Viewer)

I am trying to get hold of a copy of the 300mm but have been offered a 400mm 2.8 does anyone have experience of both for some comparison ?. If I can't get a good one I may go down the new route for a mk ii. There is a couple of grand difference between the two new but would the extra be worth it. I wouldn't be hand holding much so the weight isn't an issue. My main targets are bird/ wildlife so the extra length would be handy. Any opinions ?
By the way I have just upgraded to a 7D will this be an issue with either lens

Which version of the 400 f2.8? There have been four versions (I thnk) mkI and mkII non-IS and then mkI and mkII IS versions. They have got lighter in weight and better in optical quality as they have gone along. Personally I would not go for a non-IS version as they do seem to be a bit lacking (both are definitely out done by the IS 300 f2.8s) and they are seriously heavy. The IS versions are both very good and I would happily shoot either of them, from what I have seen the improvement for the mkII is bigger in the 400mm lenses than in the 300mms.

One thing to remember is that the 1st version canon brought out(non is)is known to suffer from its focus motor going faulty.Canon don`t repair/can`t get these units anymore the same focus unit is found in the original 300mm 2.8 as well.

Cheers.

Steve.

I think the decision for most is around wieght and not sharpness. Even the latest version is 4kg. The 300 is around 2.5kg. For most this is the difference between handholding and lugging around a tripod. If you are lugging around a tripod then most would get a longer lens to begin with...

I used to have a Canon 400 F2.8 (original version) and it was a superb lens - the newer models are even better. I currently use a Canon 300 F2.8 L IS Mk1 and it is also superb. I wouldn't worry about sharpness in any version of Canon's 300 or 400 mm F2.8 lenses.
However you state that you want the lens for "Birds/Wildlife". For smaller birds neither lens is ideal, though both take extenders well - a 500 or 600 mm (even 800 mm) may serve you better. The 300 mm F2.8 (with extenders) makes a very portable bird/wildlife setup, it is not perfect but it allows me to get to places where my long lens + tripod etc is simply too heavy.
The bigger Canon lenses are great but they do restrict your mobility so I think you have to decide which direction you want to go, I love my Canon 800 but it means I am carrying a lot of weight! Even the 400 F2.8 Mk2 is only 650 grams lighter and your overall load would be fairly similar.
If you need to move about I would look at the 300 mm F2.8 (any version) or a 500mm F4 IS (Mk1 or 2). If you are going to sit and shoot then the 500 mm + options are probably better.
Just my thoughts.

If you are not bothered about weight and will not be hand holding then I would certainly go for a 500/4 IS (or even 600/4) in preference to anything shorter.