Both paint brands offer great products, but the painter might want to use Sherwin Williams because maybe he has more experience with the particular paint he wants to use. Maybe he's never used Benjamin Moore and doesn't feel comfortable using a new product on a large project. Pricing is probably a factor too. Most contractors get discounts on Sherwin Williams paint. Discounts can be as high as 40 percent off. Some private paint stores that sell Benjamin Moore products offer discounts too. I would ask the painter what Sherwin Williams product he wants to use on your home. Make sure it's one of their premium paints and not a cheap, low quality one.