There has been some discussion about what the CGC and TDI are, and I did some looking around and made some phone calls to get it straight.
The CGC is a test done by the AKC. It is considered a title and goes after the dog's name.
TDI is a certification, not a title. It has to be renewed yearly and can be taken from your dog. TDI uses the CGC as the beginning of their test, but you do not necessarily have to get a CGC to be certified - meaning you can get TDI certified but never send in your CGC paperwork. You will actually complete the necessary parts of the CGC though so I don't know why you wouldn't do the last step and get the title but who knows. TDI can require you to re-test if they decide that your dog needs it.