The international norm for e-passports, which is set by the International Civil Aviation Organization is the face. The benefit is that all the passport agencies have a face, so it's a relatively simple thing to do.
Those countries that are adding a second biometric to their passports tend to be using a fingerprint as opposed to a retinal scan, I believe, in part because it's easier to collect. Not everybody can give a retinal scan. Some people can't give fingerprints, but I think they're fewer than the number who can't give a retinal scan.
The benefit of a fingerprint biometric is that there's a database you can compare them against. Police agencies, for example, have fingerprints on record. You're only sharing the retinal scans among those people who have collected them for that purpose. In terms of interoperability of information, which is what you want to have at the border, because the border people want to know that you are who you say you are, those that are going there are going to fingerprints on a massive scale as opposed to trusted-traveller programs.