I used the combo of APS-C with good AF (7D) and FF with mediocre AF (5DII) for a while. I'd recommend getting the 5DIII to have FF and great AF in one body.
A common concern is the loss of the 'reach advantage' of the crop sensor. In my experience, that only matters if you print very large or severely crop your APS-C images. An image from the 5DIII, cropped to the FoV of the 7D's sensor, will give you an 8.6 MP image (fine for 16x24" / A2 prints) with equivalent IQ at low ISO (up to 800) and better IQ at higher ISOs. You'd also be going from 8 fps to 6 fps, but getting a higher hit rate with the better AF of the 5DIII.
Hi Neuro, great to hear of your experience going down a similar route with the 7d/5d mkii. From the last few posts i'm leaning towards the 5d mkiii, i'm not really hearing a lot of pro's for the 7D/6D combi route. I'm going to need some good ideas to appease my wife, especially when my brother in-law is into photography so will no doubt blurt out the real cost of a 5d mkiii.