Same problem for me. I shoot motorsport and wildlife mostly. I had to sell the 500f4 and 300 f2.8 and made a nice profit, sold some other gear and pared down what i own.
I studied my meta data and looked to see what i actually shoot at. What i discovered is, i used the 500 a lot, never used a converter for extra length, shot at f4 a lot mainly due to less than impressive high ISO performance from the 7D (bought a MK4) For the 300 i never used f2.8 a great deal, shot only motorsport with it, hand held, and on occassion used a 1x4 converter. So in looking for a new lens im after.....
A lens that covers 300 to 500, will be shot at f4 a lot but on occassion steped down a little, that i can attach a full frame 1DS3 to a MK4 or 7D also for extra length. The 200-400 fits the bill perfectly.
So id think what you take pics of, study the meta data, do you need that extra stop of light especially with the new cameras ISO performance, do you need a bit more bokeh, will your eye notice any slight differance in sharpness between the lens's at sizes you print, will you use converters. Lots of questions but for what i do, the new lens fits the bill perfectly. And i wont miss shots changing my lens. I can also bet my savings it will be pin sharp with the converter.
That said......if its to expensive ill buy a new 500 and a used 300 f4.