There isn't a large difference in practice between the two methods. In either case to revoke access you need to change the authorization data attached to your account. In the case of steemvoter you have to change your key. In the case of steemian you have to remove steemian from the authorization list. It is much the same thing.
You could argue that giving your key to steemvoter increases the risk of key disclosure but at the same time authorizing steemian to act on your account means you are exposed to steemian's key being disclosed.
There is, however, a major difference in usability and user-friendliness since web users have no way to add or remove steemian from their authority list, but they do have the ability change their keys. If I were implementing a service like, for usability purposes given the current state of the steem/it tools I'd probably use the steemvoter method.
hm... thinking about it, i suppose that makes sense because you hold the higher keys, and could reverse any changes. edited to reflect
its not necessarily an overwhelming concern with something like SV (though it would be with money changing hands and the active key), but one other potential difference i see is that you can (i think) see who did what with authorizations
@marcgodard here. This is why I choose this method. I saw it as less intrusive. I also make sure that deleted accounts removes the keys and are never used again.