I spent my cash on a s/h mk2 (not even the N version) and it's brilliant. Obviously I would say that, but for a shade over £600 it is an awful lot of camera. It had 52,000 actuations. The 7d new costs about £1200 from various internet retailers, and a used 1d mk3 usually goes for about £1500-1800.
The mk2 is let down by five things, in my opinion.
1) Cumbersome menu operation, however you soon get the hang of it.
2) Screen not good enough to edit on the fly (image impact = nil)
3) High ISO performance ropey vs current generation of cameras. I don't like to go beyond 400.
4) JPEGS soft vs current generation. I shoot RAW.
5) Heavy battery and ENORMOUS charger.
Note I do not mention that 8.3mp is an issue.
Only you can decide if these things (+ 2mp) are worth an extra £1000 for a mk3. For the sake of completeness, I should mention I have never even touched a mk3, but am considerably better off for it. Nor have I ever set eyes on a 7D. I recently fell over on the ice, on top of my mk2. I bust a small piece of plastic off the battery, and the camera continued to function perfectly. My previous camera, a 50D, would have broken into about 4 separate pieces, and I would have cried a lot. I suspect the 7d would also have suffered greatly, likely terminally. The build quality of the 1 series is superb, however they weigh about twice as much as any other Canon camera. Those extra 700g or thereabouts are noticeably heavier, both to carry and to aim.
I changed from a 50D to a mk2 because I wanted the AF system and the 45 AF points. I also [really] wanted robustness of the body. I tend to bash trees, fall over, use it in the rain, lay it in the mud, sand etc more often than some. This became an awesome decision after only a few months when I fell on it. Prior to my 50D (which amazingly survived me) I wrecked my 30D by dropping it.....
So, £600-£650 for an awesomely-built, fully-featured pro SLR, admittedly lacking in user-friendliness and a few mod cons. But £1000 still in your pocket to buy that 400 f5.6 you always wanted, and still have change. Easy.
Jonathan
PS I have no doubt that the 7D is brilliant and takes great images in the right hands. But so does the mk2. And it's better made, tougher, cheaper, will last longer, won't depreciate as quickly, and I have no substance whatsoever to back this up but the 5 year old AF is probably still better. Oh, and the shutter makes a great "ker-lunk" sound which is highly satisfying.