Knowing what I know now about the way that wars of national liberation go, I would have picked the Americans. However, at the time, I would likely have picked the British.
Today, historians know that countries like Great Britain often have a very difficult time winning wars like the American Revolution. The larger countries have less incentive to fight and their people tend to tire of the wars and the expense in money and lives that go with them. This is, for example, how the US lost in Vietnam. Therefore, the Americans' psychological and motivational advantage made them more likely to win.
However, at the time, this trend had not really been established. I would have been more likely to think that the British would win. This would have been because of the size and professionalism of their army and the fact that they had the strongest navy in the world while the colonists had no navy at all.