Sub domain? Micro site? What's the best solution?

[WhitewallGlasgow]

My client currently has two websites to promote their art galleries in different parts of the country.

They have bought a new domain (let's call it buyart.com) which they would eventually like to use as an e-commerce platform.

They are wondering whether they keep their existing two gallery websites (non e-commerce) separate as they always have been, or somehow combine these into the new domain and have one overarching brand (buyart.com).

I've read a bit on subdomains and microsites but am unsure at this stage what the best option would be, and what the pros and cons are. My feeling is to bring it all together under buyart.com so everything is in one place and creates a better user journey for anyone who would like to visit.

Thoughts?