Respecting your recognition of the geographic differences, it might be worth mentioning for other readers that Harris Tweed is created in the Outer Hebrides Islands of Scotland from local (Scottish) wool. http://www.harristweed.org/
It is a carefully controlled production situation.
Donegal Tweed is supposedly created in County Donegal in Ireland. I am not aware whether or not there is an Irish counterpart to Scotland’s Harris Tweed Authority. It seems to me that Donegal Tweed is relatively abundant in its availability. That availability begs the question (for me) of the wool's origin if not the location of the looms.
Mr. Carl Evans of the Harris Tweed Authority might respond in a knowledgeable fashion to the subject of the technical differences. I expect he might
extol the merits of Harris Tweed in his remarks.
It seems to me that Harris Tweed is by its very nature much more of a genuine cottage industry. Nevertheless, high quality tweeds of any verifiable provenance are a wonderful luxury addition to anyone’s wardrobe. I have both tweeds and find no appreciable difference in the garments beyond the tailor’s skill.