Both Canon and Nikon are somewhat stagnant right now, in terms of the features on their "new" bodies. I put "new" in quotes because the photographic aspect of the cameras has mostly remained unchanged from their 2010 and 2011 bodies. There's really nothing new of note -- nothing to get excited about. The biggest push in SLR cameras right now is increases in video abilities. So if you're like most photographers, you probably won't care all that much about the latest-and-greatest offerings from the big two camera manufacturers.
The differences between the Canon 5D Mark II and Canon 5D Mark III -- much like the Nikon D3s and Nikon D4 -- are minimal.
In the case of the 5D bodies, comparing EOS 5D Mk II vs EOS 5D Mk III:
- the resolution is almost the same
- the DiGiC processor is almost the same (one generation apart, which doesn't mean much)
- the max frames per second (fps) went up from 4 to 6 -- which is honestly not much of a boost; still a far cry from 9/10/11 fps
- there are more focus and meter points, but many photographers find these to be overkill anyway
- the viewfinder went to 100% coverage which is nice, but not all that special
- an SD card slot was added -- however SD cards are very unreliable (compared to CompactFlash), so they're best avoided
- more video resolutions and framerates (1080p30)
The ISO increase was the only major stand-out.
The MkII was a 6400 ISO native max, while the MkIII moved to a 25600 native max, similar to the higher end Nikon bodies, which have been outperforming Canon low-light (high ISO) for almost 5 years now. Canon is starting to catch up to Nikon again. But unless you're shooting in caves, horrible indoor settings (weddings, home events), or sports (especially crappy high school and college gyms/stadiums), then this may not be an upgrade-worthy feature for you. I mostly shoot in horrible lighting, so I've always been the person who pushes film and sensors as much as possible.
I've shot the 5D and 5D MkII, and both are excellent cameras. There's almost nothing I cannot do with either of those. The MkIII is just more candy coating, excluding the ISO boost.
If I had a MkII, and was not regularly (daily/weekly) shooting in horrible light, I would NOT upgrade.
In fact, as I was mentioning to some fellow photographers on Facebook last week, the release of the MkIII makes for a good time to get discounted used/new MkII bodies, as well as watch the pricing of the original 5D drop even further. Those cameras are excellent, and I've always found it silly how technology-leaning camera geeks think previous models of DSLRs are "obsolete" or otherwise outdated and useless.
So it really comes down to this: What do you shoot? (And how often.)
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Above all else -- for you and anybody else reading -- ALWAYS buy from a reputable camera store. Jessops is good, and I'll assume you're in Europe since that's a European store (mostly UK). More quality/authorized camera merchants include
B&H,
Amazon.com,
Amazon.co.uk,
Amazon.de, Wolf Camera, Ritz Camera and
Adorama.
Also in UK is Microglobe, which has a mix of gray-market and authorized UK bodies and lenses. I've shopped there before. I mostly used them for gray market lenses, to shave a few dollars off the prices. Plus they seem to be in stock on certain older lenses when nobody else (in the world!) was. Just be aware that gray-market carries no warranties.
