SmartAdmin comes with a fully customized grid system catered specifically for building form layouts. Its not technically "better" than the bootstrap 3 built in grid system, but rather more simplified for rapid form layout and faster development. Idealy you would use either the bootstrap grid or the smart-form grid, when building your form layouts. It is important not to mix elements from two seperate classes as it can cause conflict.
You can explore some of the layouts made using Smart Forms grid system or if you are not comfortable with this grid system you can simply stick to the default Bootstrap forms