"If it comes down to two great rounds in a hunter class, one ridden by a professional and one by an amateur or junior, are there differences in the way the two rounds are judged?"
Judges will notice an experienced professional rider or trainer versus a less experienced young rider or amateur adult as soon as he/she enters the show ring. Some class divisions are designed to separate non-professional (amateurs and juniors) from professional riders (trainers). But some hunter classes allow both, and does the judge take into consideration who is riding the horse
Watch and listen to the answers from some of the top hunter judges in the country. They have presided over the highest level of hunter shows from the east to the west coast, such as Palm Beach, Capitol Challenge, Harrisburg, Washington, HITS, Toronto Royal Winter Fair, Devon and many others.  They will tell you what wins.