Smartphones win this hands down since they are a complete time sink, personal computers have helped business to become way more efficient, but smartphones are a plague when it comes to productivity, while I have no data at least in my job I will speculate that about half of the time my coworkers are just staring at their smartphones and not working, it does not matter if they are just talking over the phone, interacting with people in social media or playing video games in my opinion they should not be paid for all of that wasted time.
It used to be very bad here with how people paid more attention to their smartphones than the work they were doing, but it has decreased to a level where it has become somewhat acceptable.
It's impossible to prevent people completely from browsing through their phones every now and then in a time where pretty much our entire life revolves around the internet.
At the end of the day it comes down to how the employer adapts to the change in how we communicate with each other. Firm rules on smartphone use is necessary, but not to the degree where people feel annoyed.
Whether you're spending too much time on your smartphone or not being able to focus on your job because you don't like the environment there, the outcome is the same, which is less productivity.