The Oxford Guide to English Usage

Scottish is now the usual adjective; Scotch is restricted to a fairly large number of fixed expressions, e. g. Scotch broth, egg, whisky; Scots is used mainly for the Scottish dialect of English, in the names of regiments, and in Scotsman, Scotswoman (Scotchman, -woman are old-fashioned). To designate the inhabitants of Scotland, the plural noun Scots is normal.