Here is a comparison of all 3 plan areas - Brentwood, Metrotown, and Surrey City Centre.
By looking at this, Surrey City Centre is much better positioned to develop into a regional downtown with a downtown feel based on its size, road network, land-use designations, and integration with its surrounding area. Not to mention its location South of the Fraser which is becoming a distinct region from the Burrard Peninsula of Vancouver, Burnaby, Coquitlam, Etc..
Metrotown's road network and urban feel suffers by the mall taking up a large chunk of its centre.
Brentwood is a very linear, big block strip along Lougheed Highway that doesn't integrate well with its surroundings.