In Ruby, should I use ||= or if defined? for memoization?

Should I use if defined?

 return @current_user_session if defined?(@current_user_session)
 @current_user_session = UserSession.find

Or ||=

@current_user_session ||= UserSession.find

I noticed the if defined? method being used more and more recently. Is there any advantage to one over the other? Personally, I prefer ||= for readability. I also think Rails might have a memoize macro which provides this behavior transparently. Is this the case?