The other way to look at it is this.
It takes me roughly 30 minutes to play an 18 hole comp for 400 xp.
It takes me roughly 15 minutes to play an 9 hole comp for 190 xp.
So it is slightly more advantageous to play an 18 hole comp for the time and xp involved, plus if you play 2 x 9 hole comps there is also the time involved in going to the menu and clicking etc.
Don't forget to play every day for the Continuous Days Play xp bonus (even a random 3 holes played and finished counts as a day's play).