I'm glad I don't have this decision to make. I have both the 1D4 and 5D3 (even the 1D3) and I use both for everything I do; wildlife, sports, portraits. I don't pigeon hole my gear - all get used for everything. Funny, I downsize my 1D4 to 8fps. I hate to admit it, but the 5D3 AF is a bit better than the 1D4 and mine haven't needed any Maf for my lenses. I can get the 1DX (great camera) if I want/wanted to, but I just did/don't want to give Canon 7k for a camera body. I just don't see it. I will just wait them out like I did with the others. They will come out with a new 1DXs or something, these folks with jump, the prices will drop and then I'd consider one.
What I've notice from when I was entering the game, I read where many stated that they didn't need high fps because they do not spray and pray and that timing was everything (sure it still is, but don't here it mention much anymore, only fps), but lately many of my readings have found that many are saying they really depend on the really high fps these days to get that 1 shot. I guess technology has changed the way many thing on this subject, just as it did with megapixels. I know everyone does things differently and are at different experience levels. When I first started, I used to spray, but overtime I've gotten better at it and that's one of main reasons I downsize my fps on the 1D4 from 10 to 8 and even then I don't use all of it. Just an observation.
To the OP I'd suggest you hold off for now if you can and wait to see what Canon does. Also, the buffer will not be an issue as state above with the appropriate memory card, especially at 6fps (Lexar 1000 card are on sale at Adorama and B&H).
Just One Man's Opinion.