I sold my 100-400 MK I to get the 70-200 2.8 IS MK II with 2x TC II, so I would never sell it, still the 100-400 II is better than the first version and so its a different story, I am not a pro as well, so yeah budget matters, and only you can decide which to keep based on your current gear and needs.
However here are somethings to consider:
Do you shoot Weddings?Portraits?Indoor Sports? if no then sell it, if yes; do you like bokeh more or sharpness? do you want better low light performance or the zoom versatility? as you may already know, the 70-200 2.8 IS II is the sharpest lens in the focal range at this price, however you can get up to two lenses to replace from the following list (135L, 85L, 50L, 35L, 50 Art, 35 Art) you can get these for low light and extreme bokeh.
- Do you care about sharpness?
Some said that the 100-400 MK I is better than the 70-200 II with extender, I guess it depends on sample variation, but that wasn't the case with me, below is a 100% crop of a shot taken with the 70-200 2.8 IS II with the 2x TC II @ 400mm 1/1000 f/5.6 ISO 400 with the 5D3, the image is straight out of LR with no sharpness added except for the default value LR does, of course it may not be as sharp as the 100-400 II - which is why I plan on getting one - but I guess its good enough, most of the time though I don't get images as good as this one, which is only caused by my lack of skill.
Yes really, how strong are you? do you like carrying that bag with both the 100-400 II and the 70-200 II?
- Which lenses do you have?
Do you have mid range zooms? ultra wides? nifty 50? this overlaps with the first point but even if you don't shoot landscapes its a must to have a mid range zoom or ultra wide zoom, you can get 16-35 f/4 IS and the 24-70 f.4 IS for the 70-200 II and little extra which covers you for the full range, you may have these and want to invest for the new 11-24.
In the end it depends on what you want and need, only you can decide.
Edit: for comparison sake I add an image taken with the 100-400 I on the 600D, I know its not a fair comparison but that is what I had at the time, the shot was taken @ 400mm 1/640 f/6.3 ISO 100