This is a bad question for two reasons:
1. It varies from situation to situation, environment to environment.
2. There will always be arguments as to which side is more effective, and there are several good points to each side. A more useable discussion might be:
Which is better? To keep your 'opponent' acting, or reacting? You can do both on either side of the fence, and I think you'll probably end up with a more useable discussion of technique than a simple "what's harder -- defending or attacking?"