The `user_id` is the per-side user id (in the above case, for Stack Overflow). The `account_id` is the global Stack Exchange account id (i.e. the id of [your Stack Exchange account](http://stackexchange.com/users/2417804/user202450?tab=accounts)).

The `account_id` is probably the best key if you're looking for a global identifier, since the individual per-side user ids will be different on each site the account is associated with.