Advantages of spray vs roller system for painting a flat
Sprayers are capable of giving a very smooth finish very quickly. That is why you will often see professionals use sprayers when finishing trim, stairs and cabinets. The negatives of a sprayer would be the amount of prep time required to start painting. You have to tape and plastic EVERYTHING. Do not underestimate the amount of over and back spray you will get from a sprayer.

Also keep in mind if the walls aren't smooth right now, spraying over them won't make them smooth. You'll need to sand them down before applying your coating. You can also get a relatively smooth finish with the right roller and pan if you spend enough time sanding and prepping your surfaces.