Design Build firms vs. Contractors - gut reno

We are in contract on a UWS Classic 6 Sponsor unit. It hasn’t been updated in years but it’s just about livable (can stay through planning but not construction). We are hoping to move a couple of walls/doors, split the dining room to make a 3 main bedroom, absorb gallery into more living space, include central air, replace plumbing and electric, upgrade bathrooms and kitchen. Meeting with contractors and design & build firms in the coming weeks. Have two small kids and busy lives. Interested in thoughts on experiences and budget when working with D&B firms vs. managing relationship with Contractor. This is not our ‘forever home’. Idea is to sell in 7-10 years.