With regards to Sutherland, it appears that after the heir apparent (b. 1975) and his brother (b.1977), there are only 3 other males in remainder, per DPB Online, born 1949, 1959 and 1967.
Unless one of them has a male heir DPB doesn't know about, in due course, the Dukedom will become extinct, and the Earls Granville will succeed to the secondary title of Marquess of Stafford.
In the case of Westminster, there's still hope, as the Duke and Duchess just became first time parents a few months ago, but as of now, the Earl of of WIlton would inherit the secondary title of Marquess of Westminster.
Brooke