The 5D3 will simply not write fast enough to stretch either of those CF cards. It is only when you get to cameras that will write really fast that the top cards show their benefits (unless you are in a real hurry to download to your PC).
I did a rough test on my 1DX and found that the Lexar 1000x 32Gb card gave me just over 50 RAW files before it slowed, my Transcend 400X 16 gb card gave just over 30. Naturally the Lexar is better but the 1DX has 3 processors (2 for this sort of thing) so the Lexar is much better in this camera. Conducting the same test on my 1D4 the cards made little difference, though to be fair, the buffer cleared a bit quicker with the Lexar.
I would have though the Lexar 600x or Sandisk equivalent would be the best price/speed compromise for a 5D3, though others may have better insights.