Yea, it does depend on how much protection you need.
Speck and like cases offer little protection, and are mainly made to protect the mac from small scratches and scuffs.
BookBook, when zipped shut, will offer a higher degree of protection, and it still protects the Mac from scratches and scuffs during use (if you keep the mac in the case of course), but I wouldn't trust the mac in this case on a concrete floor.
Sleeves offer a similar level of protection to the BookBook, but there are quite a few sleeves/bags that give much more padding than the average case, making them very protective.