Based on your experience. Do you think it's better to upgrade your ISO 9001;2008 to 9001:2015 using internal resources or through consultant,why?

it depends on the quality assurance manager in the organization if he has a good experience in building QMS. He can get the 2015 standard and he can find over the net the differences bet 2008 and 2015 standards and adjusts accordingly. Then he call the certifying agent to make the audit and gap analysis and certify him.

It is always a good strategy to use internal resources to implement any management system in the organization because no external expert knows better than you about your organization. Consultant gathers all the information from you and give it back to you.