Yup you're right, sorry to OP and thanks for highlighting. The 70-300mm L is a more recent lens. From what I have seen, quicker AF, better IS than 100-400mm. You're "used" to an effective reach of ~300mm, so I would add another option which is simply the 1.4x extender to give you the reach you had. And I've seen a review which said quality of the 70-200 with 1.4x and 70-300mm was close. If you don't need the range of 300mm, then the 70-200 without the 1.4x converter is sharper. The 1.4x might also defer your decision until a possible 100-400mm MK II appeared.Not a Canon extender, no, Canon "fixed" that. It does with the Kenko Pro range. Alas not had chance to compare with the 100-400 as mine is alas a rubbish copy. Others however on this forum have good copies so I think that I'm in a minority.Why not get the Canon Extender 1.4x II or 2x III?
The 70-300 L does not accept an extender.
@OP - You're 200mm on the 50D gave you effective 320mm, with 1.4x converter then it gave you 448mm. If you need that sort of reach, then is it not more of a compare the 100-400mm with the 400mm f/5.6 and keep you're existing 70-200?
I Think criza meant an 1.4 extender to the 70-200f4. Which would actually make the fov similar to an 50d without extender...
If you need 400mm reach, and want flexibility, then the 100-400mm is your option. I've not seen comparison on 400mm f/4 vs the 100-400mm if reach is your only concern and you have flexibility in positioning.
Of course, if you have a friendly dealer, then I am sure you could try the lenses outside their shop with moving vehicles and see which you prefer...?