Canon seems to be flinging new bodies into the market at a very rapid pace right now. May be a sign of production getting itself sorted out after last year. Even so, if it hasn't been announced and released who knows when an updated product will appear these days. It will take a bit of time to recoup all the development costs of everything recently brought into the marketplace.

5D3 is an excellent body. Have replaced my 7D and 5D2 bodies with it. Superior AF by far, amongst other improvements, with IQ slightly ahead of 5D2 and no loss of resolution. If you can swing it financially and want to take the leap I'd recommend it highly.

If most of what you photogragh is relatively still, the 5D2 still works very well. Just doesn't like faster moving targets.

The differences in the 18 MP C sized bodies are quite well covered in Bryan's reviews if there are features missing from your XSi that have become must-haves. 7D is the only one of the lot I had. used it for the AF system, not image quality.