Each of these slogans has a slightly different impact. There really was a 'We are Juan' campaign to stop a deportation, though the link seems to be gone. I personally like that slogan better than I am Juan, but I like We are all Juan best. It is the closest translation to the zapatista solidarity 'Todos somos Marcos' slogan that may have started this wave, but it also seems less appropriative. It does not imply I am exactly like Juan, or totally know his experience, but rather that I stand with him, if you attack him, you attack me. An injury to one is an injury to all.
I've written before about the drawbacks of 'I am X person under attack' type campaigns, such as the current ''I am Bradley Manning campaign - though it's very different when Daniel Ellsberg says it, since he really did live through a very similar persecution (please sign Daniel's petition to support Bradley).