What exactly is the difference between handling login success/failure in the Meteor.loginWithPassword callback vs using the Accounts.onLogin and Accounts.onLoginFailure methods?
The largest difference is that Acconts.onLogin and Accounts.onLoginFailure methods can be called on the server while Meteor.loginWith<Service> methods can only be called on the client.
With those methods you can create application logic that handles successful/failed login attempts, e.g send a notification email to the user that either he/she or someone else has failed to login to his/hers account five times in a row.
What you do with these hooks are up to you, but they are available if you need them.
Makes sense. Good to have options 
Thanks!
Accounts.onLogin works with client also now 