70-300 f/4-5.6L or 70-200mm f/4L with 1.4x Extender?

Hi, yeah i have pondered the sale of my 70-200f4 and getting the 70-300 instead. But if i have to start selling, i'll sell allthe teles and get the new 200-400 (if its lighter than my 500f4)! me 500 is getting awlfully heavy these days...;)
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