To a point, this is a personal preference but there are some things to consider.
Shelves make it easier to spot and select products as they are all in the open. The downside is that unless you keep things nice and tidy, open shelving has the potential to look messy.
The other issue is that uncovered shelving has a tendency to collect dust and hair as it's blown around the salon making them harder to keep clean and sanitary. For example, California code dictates that items like non-electrical tools, smocks, towels, and linens need to be kept in closed cabinets or drawers.
Drawers are great because they help keep out dust and other airborne debris while hiding your pile of jumbled hair clips away from the eyes of the public. Keeping your space looking neat goes a long way to your clients believing that your space is clean--something that MTI College emphasizes.
However, because they have to slide, drawers are often not as strong as shelves and can be harder to see exactly what you need at a glance.