Both online and offline marketing are efficient. You are right when you say that offline marketing takes longer. It does. If you mail out a salesletter, it will take several weeks before you can gauge whether it's been successful or not. It's also a lot more expensive. Sending out letters or postcards, for example, is a lot more expensive than sending out an email to your list of subscribers.
In order to be successful with offline marketing, you've really got to concentrate on a list of qualified prospects.
Which one you should use really depends on the product or service you're trying to sell and the market you're trying to reach. If you're just starting out and selling low-cost products and/or services, I'd suggest sticking to online methods. It's a lot more targeted and provides results much faster.