I have the 70-300L and the new 100-400L II. Advantages to both. The 70-300 on my crop body is close to the 100-400 on my full frame for actual zoom range. The 70-300 is very noticeably lighter, shorter, more comfortable to carry and takes awesome photos. The 100-400 on my gripped 6D is better with stabilization, slightly better photos but normally not really noticeable difference for my type hobby usage. I really love both lenses. Originally I was going to sell the 70-300L to recoup some money but just can't part with it.
Guess I am saying if you can keep it for a while and still purchase the 100-400L II, you may end up like me and find uses for both. My wife dislikes the 100-400 due to weight and size but never takes the 70-300 off of her 60D.