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Thanks for the respons. Responsive design is probably the best option with the best usability users.

Because domains with a mobile version have better rankings in Google then domains with only a deskop version we're considering a mobile website. My web developer says that a m.domain.com is duplicated with the domain.com, and he recommends a responsive design.

What is better for SEO positions in mobiles devices. A m.domain mobile website or a responsive design. What are influential factors?