Brush selection is a lot about personal preference. An extra soft brush allows you to hit the large surface spaces of gel coat or paint, without scratching. Although for these surfaces a lot of people like to use a mit. A regular yellow soft brush is ideal for all around cleaning. If you're only buying one brush, this is the one. Typically, a soft brush is enough to get the deck and fish boxes clean and is safe for shiny surfaces if you keep the brush clean and the boat well maintained. Keep sand, dirt and grime away from it as much as possible. Keep a stiff brush or a hand brush on board for the gritty cleaning. Shurhold's stiff shite brush is specifically designed for heavy chemical use because it is resistant to acid. But be careful! Because it's stiff it can fling that acid or chemical on you or other sensitive surfaces of your boat. Follow up cleaning with a soft hand brush for those hard to reach places!
Scotchbrite scrubbers are best when used on teak and bottom paint for light scrubbing or surface prep only.