Which tele extention option is best?

One option is the Kenko 2x Teleplus Pro 300 DGX teleconverter. It will maintain autofocus even on a non-1D series body. The older DG version will not work the same way so make sure you get the DGX version.

Your other option (and what I think you should do) is get a 1.4x teleconverter (either Canon, Kenko, Tamron, etc - any of the pro series teleconverters). Then crop from your 1.4x. The two-stop loss with a 2x is a lot - especially on an f/4 lens.