The major upside to having professional jurors is that they would be more likely to deliver good verdicts. In our current system, people are asked to understand the law (particularly in civil cases) without any training. They are given brief instructions by judges and then expected to apply complex laws. Lawyers have to have whole classes on various areas of law. We should not expect jurors to understand those laws on the basis of one short lecture from a judge. Therefore, having professional jurors would give us better verdicts more clearly based on the law.
The negative aspect of this is the loss of citizen participation. Jury trials exist so that common people can have an impact on the legal system. The Founders did not want Americans to be tried and judged by legal elites who might lose touch with the common people. They were afraid that this could lead to tyranny. We may not be in danger of tyranny, but we would lose an important aspect of our citizenship and our rights if regular people no longer served on juries.