Ad-based versus direct monetization: Which one is better for you?

comparing the leaders in both monetization methods is easy - what about the results in general?

I'd guess the answer would be 'none work': you rarely break even with ad-based businesses (since as the traffic grows, there's a tendency of getting more maintenance cost than revenue per user) nor with paid services (since it's hard to attract paying users when you have so many people offering the same stuff for free)...