Gallowglasses are far superior to Highland Clansmen.
Gallowglasses get 8 for charge bonus, Highlanders get 6.
Gallowglasses get no defense bonus, Highlanders get a -3 penalty.
Gallowglasses get 2 armor, Highlanders get only 1.

This, I guess, is offset by the fact that Highlanders can be built by any Catholic factions with just the first castle (fort, I believe), whereas Gallowglasses require the faction to be English and requires a swordsmith.