People have been considering the pros and cons of professional juries going at least as far back as Classical Athens when Aristophanes staged his satirical play "The Wasps" on precisely this topic. Let's look at some of the pros and cons of having professional juries in today's legal system.
An obvious pro is that a professional juror would have a better understanding of legal nuances than the average citizen. They would be more likely to reach a decision based on legal requirements. It is not unheard of for a jury to reach a verdict based on emotional decisions or a lack of legal understanding. A professional jury would likely eliminate this problem.
Another pro to consider is the unwillingness of many citizens to serve on a jury. It is sometimes difficult to fill a jury bench with willing citizens, since being part of a jury is a significant disruption to one's life. Particularly when it comes to lengthy trials, many reluctant jurors will not actively participate in deliberations, or they will be less engaged during the proceedings. A professional juror will likely be more engaged in the process, as this would actually be his or her job.
A juror is supposed to weigh only the facts of the case in which they are taking part. A professional juror would participate in many trials and, as a result, have many examples of which to compare their cases. While they may not do so consciously, it is plausible that the circumstances of a different trial could influence their verdict on unrelated proceedings. This is partly why normal jurors are excused from serving on a jury for several years after they participate in a trial.
A jury is supposed to consist of one's peers. A professional jury would not be part of the same random selection process as a normal jury. It would likely consist of people with a similar educational and socio-economic background. As a result, it would likely be less diverse in its composition and opinions.
A non-professional jury would be more likely to practice jury nullification than a professional one. This is a powerful, although seldom, used tool to keep a government's power in check. A professional jury, being part of the government, would probably be less likely to implement jury nullification.