There are a couple more points to consider. If you only have to carry it a few yards, from the car to the location, portability is less important than if you are backpacking into the wilderness. Don't kid yourself, either. They have to be comfortable, and not dig or bind. If you sling the bag diagonally over your shoulder then an adjustable strap is a good idea.
Some people prefer padded straps, others prefer neoprene which flexes and takes some of the strain.
Accessory pads, which fit onto a shoulder strap to prevent it slipping off, are available from at least two companies, Domke and Tenba. Fasteners should be secure, but easy to use. It is all too easy to leave a bag unzipped or unfastened, leaving equipment vulnerable to loss, theft, or the elements. Think about how you will be using the bag and whether the fastener will be convenient. This can be overrated.