I'm thinking about the differences between physical authentication tokens such as RSA's synchronous physical tokens or asynchronous challenge/response, like Google's gmail direct-to-phone codes.
At the highest level of abstraction, are there reasons why a synchronous token will be better than an asynchronous token and/or vice-versa?
(The potential to introduce implementation-dependent security flaws may be a specific con to be considered at this high level of abstraction: maybe synchronous systems are harder to implement over asynchronous ones. For example, RSA tokens need to be physically secure to protect the seed as opposed to a phone, which doesn't necessarily have to be until the response arrives and then only while the response is still potentially useful.)