Some people report the autofocus system on the 1D3 as being better than that on the 7D, others (fewer) report them as being about the same. Similarly, the 7D and 1D3 are pretty much neck and neck in terms of high ISO performance, with the 7D's images recording more detail but containing more noise that, if you downsize to 10 megapixels (the 1D3's native resolution), winds up getting you something that is essentially indistinguishable. The 7D is capable of recording significantly more detail at lower ISOs, however, so if you intend to print very large, the 7D is likely to be better suited.
You get go wider on the 7D with the appropriate lenses (e.g., Sigma 8-16) than you can on the 1D3, and similarly you get more "reach" from longer lenses, and the body itself is lighter and smaller and thus a bit more portable, but at the same time it doesn't have that 1D series tank-like build that lets you use it simultaneously as a camera, a hammer, and a defensive weapon. 
I believe the 1D3 is a couple of hundred or so more expensive than the 7D -- not enough to really make the decision in and of itself, in my opinion, although the 7D will be capable of using all your current lenses while the 1D3 will vignette with your 17-50, so there is some savings to be had there.
I'd decide between the two based primarily on what you want out of the ergonomics. If you want something larger and heavier (to better balance larger lenses, for instance), the 1D3 is for you. If you want something smaller and lighter, the 7D is probably what you want. If there is any image quality difference between the two, it will be so slight as to hardly matter.
It's a difficult decision in the end, and you'll wind up with a stellar camera in either. Because they're so close to each other in terms of overall capability, if you're budget limited, I'd go with the 7D (primarily because you'll be able to save on glass).
Canon 60d; Canon 40d;Sigma 18-35mm f/1.8 DC HSM:Tamron17-50mm f2.8;50mm f1.8II;70-200mm f2.8 L;canon 580exII .
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