Should I use a hard shell case or a sleeve for my MacBook Air?

A sleeve is better for both overheating and scratching. Hard snap-on cases trap heat, turning a fanless MacBook Air into an oven and causing throttling or a slower laptop. Sleeves protect from scratches and dings during transport without blocking cooling during use. A thin neoprene or felt sleeve from Tomtoc or WaterField does the job without cooking your machine.