At the World Publishing Expo 2013, currently taking place in Berlin, Miller demonstrated the advantages of on open system, showing the Guardian's real-time analytics platform Ophan and referrals from social media.
"If you are behind a paywall, you can’t be consumed in all these places," he said.
Taking a different viewpoint, Döpfner explained the success of the Die Welt paywall. He said 47,000 have signed up for digital-only subscriptions, and there are 27,000 bundled subscribers. Unique browser numbers did not drop when the paywall was introduced in September 2012, "which we fully expected would happen", Döpfner explained.
Miller pointed out that the Guardian is "not anti-paywall", explaining how the business constantly re-evaluates. "But we are going to have to ask our readers for more money in the future," he said.
He also referred to the fact that there maybe "a trade-off" in payment for privacy in years to come. Readers could potentially opt to pay if they do not want their data tracked, for example, or could use a site or platform for free if if they do not mind. "But that’s years away," Miller added.
The current focus is on growling global reach, Miller explained, referring to recent expansions. The Guardian launched in America in 2011 and in Australia earlier this year.
The Guardian has transformed from being the second-smallest national newspaper in UK, Miller said, to the third biggest news outlet internationally thanks to digital.
Both chief executives were keen to stress the importance of quality journalism.
“The Guardian lives or dies by its journalism,” said Miller. The task ahead is "not about saving newspapers, it’s about protecting and growing journalism”, he added.
Miller was asked by moderator Ken Doctor, author of Newsonomics, to score the news organisation's progress in digital transition, giving it a score out of 10.
He gave a score of nine out of 10 to readers in terms of their digital transition; he said the Guardian gets a score of three out of 10 in terms of where Miller wants the business to be; he gave the industry as a whole a score of two out of 10.
'Upbeat' about the future
Miller said he was "confident" and "upbeat" about the future. "It’s about innovation and following the consumer through the journey," he said.
The Guardian website had 84 million global unique visitors in August, according to the latest audited statistics from the Audited Bureau of Circulation. It recorded a loss of £30.9 million in the year to March, an improvement on the £44.2 million loss last year.
Miller urged delegates at the conference to experiment and try new things, citing VICE and BuzzFeed as newer outlets that are getting attention.
"Is publishing a sustainable business model?" Döpfner asked. If the question is related to print the answer is "no", he said, but if digital platforms are added the answer is "perhaps".
“We need to emancipate a newspaper from paper,” he said, later explaining that the business gets 47 per cent of its profits from digital.
Döpfner also said publishers need to "refocus priorities".
"We’ve been spending time focusing on how to save it", he said. As stressed by Miller in his presentation, Döpfner said that "we need to make sure we have the best possible journalism."
"We need to invest in journalism," he said. "It’s the genetic code of publishers."
“A good story is a good story is a good story, independent of the distribution channel. If we do it right the best times of publishing are ahead of us.”
Journalism.co.uk is at the World Publishing Expo in Berlin. Follow @SarahMarshall3 / @johncthompson / #wpe13 for updates. The 'live notes' of today's sessions are at this link.