Michael Barthorp (author of The Jacobite Rebellions 1689–1745. Men-at-arms series #118, 1982, Osprey Publishing) proposes that the clans which supported the Jacobite cause tended to be Roman Catholic and Scottish Episcopalian, while clans which tended to be Presbyterian sided more with the British government. A search of the online vital records on the Scottish government’s website, Scotland’s People, reveals that people with the surname Buchanan are predominantly Presbyterian. This does not necessitate that Clan Buchanan would have sided with the British government at the time of the Jacobite Risings.
From what I have been able to glean, people with the surname Buchanan were on both the British government and Jacobite sides. (My findings have been summarised on the Wikipedia, Clan Buchanan webpage, https://en.wikipedia.org/wiki/Clan_Buchanan).
Because Clan Buchanan had been without a recognised leader since the 1680s, it is unlikely that the question of ‘What side of the Jacobite Rising did the Clan align itself?’ can be authoritatively answered. Nonetheless, I think that on the balance of probability, more clan folk would have been aligned with the British government.
For some people, these are fighting words because they want the Clan to be aligned with the Jacobites. As we are nearly 300 years on from Culloden, I am personally ambivalent as to the how most of the clan folk aligned themselves; however, as a matter of Clan history, I would love find the truth of the matter. If you know of any books or other sources that would shed light on the matter, please reply to this post and let me know.