After much deliberation I recently went for a well looked after second hand 5D and I don't regret it at all.
It depends what your priorities are. If image quality, 5D. If you want video as well, 550D. Either will take great photos, but for me the viewfinder, full frame sensor and fantastic IQ made the 5D the right choice for me - it's just a great photographic tool (but I'm not interested in video).
The price difference between the 5D and MkII is painful.
After much deliberation I recently went for a well looked after second hand 5D and I don't regret it at all.
It depends what your priorities are. If image quality, 5D. If you want video as well, 550D. Either will take great photos, but for me the viewfinder, full frame sensor and fantastic IQ made the 5D the right choice for me - it's just a great photographic tool (but I'm not interested in video).
The price difference between the 5D and MkII is painful.