if statements are nearly always bad, because they don't communicate anything and they make the execution flow opaque. Will this method do what it says it will? Who knows! Will this statement be executed? Well, unless some condition is met along the way! Sometimes they are necessary, as in guard clauses, but I find avoiding them as much as possible leads to far more maintainable code.
If, and only if, my code is too slow is your concern relevant. That happens so rarely I find it not worth spending time on.
(Edited original post to clarify that I was not talking about performance.)
If the inheritance already exists and make sense, adding another overridden method may be a win. The thing I'm arguing against is introducing a new class hierarchy to remove an if statement. Inheritance is a big gun and you shouldn't use it unless it significantly cleans up the code.
In this particular case, extending User to get TwitterUser is a particularly bad example because it's adding an is-a relationship when has-a is better. After all, users may have both twitter accounts and email addresses, and these can change dynamically (they can edit their notification settings). Unthinking use of inheritance is far worse than extra if statements.
If, and only if, my code is too slow is your concern relevant. That happens so rarely I find it not worth spending time on.