"A Scottish preference?"
Quite possibly, harking back to the Auld Alliance and a preference for
all things French. Debrett's spells it with an 'e' in the family's
entry (a proof of which Lord Bowmont's father will have seen, or had
the opportunity to see at any rate).
Debrett's entries for the Scottish dukes of Argyll (Marquess of
Lorne), Atholl (Marquess of Tullibardine) and Montrose (Marquess of
Graham) all use the e spelling.
And the entries for the Scottish marquesses of Huntly, Lothian and
Queensberry all use the e spelling.
There is a Complete Peerage appendix on the spelling of the degree,
which concluded that an e was now preferable (I don't have the volume
to hand; I think part of the reasoning was e was more 'English' (read
British) sounding and the i spelling had been debased as there were so
many French men about who claimed to be marquisses - can't remember
just now the popular perjorative French phrase used to denote this).
But of course it's a matter on which each title holder may decide,
since both spellings are correct.