Upon review, I think I can safely say that there is no major difference between Canning's foreign policy and Castlereagh's. This is because all foreign policy is essentially the same thing, currying favor with other nations for favorable tariffs and military alliances. Whatever the means to reach that end are trivial.
Yes, there is a difference between Canning's foreign policy and Castlereagh's, because at the time, people were very divided about what policy to follow. They disagreed so vehemently, that it ended up in a duel. They must have had strong opinions if they were willing to duel over the issue. They disagreed.
Yes, there was a difference between Canning's foreign policy and Castlereagh's, because there was enough of a disagreement to represent major differences in foreign policy. Many people were divided on how to best handle the conflicts, and Canning and Castlereagh stood for those differences. They each thought the other was in conflict with their own beliefs. Yes, there was a significant different in their foreign policy.
Despite their famous duel, both Canning and Castlereagh felt it was wise to send British troops to the European mainland in order to stop Napoleon. At the time, the French emperor was taking over half of Europe and needed to be stopped before he invaded Britain. Both men felt they should send troops, they just didn't agree on where those troops needed to be placed.
Both men served Britain in a time of tumult. England tried to win back its colonies lost in the American Revolution and burned Washington DC to the ground. Then the British had to turn back Napoleon and Waterloo. It was too much to handle and the era marked a downturn point for Britain's influence on the world. Both Canning and Castlereagh felt Britain's military might was needed to preserve Europe as a whole, they just felt like going about it in different ways.